Aristotle’s Theory of Conception

With this module we shift from what was, at least in Plato, a rather abstract discussion of love to a much more concrete discussion of sex – and of sexual anatomy in particular.

Aristotle and Plato from Raphael’s The School of Athens

Aristotle was Plato’s most famous student, but he diverged from Plato’s teachings in many ways. In particular, while Plato is a rationalist (his arguments are abstract) and wrote in a highly literary form, Aristotle was an empiricist (his arguments are based in observation) and wrote in the form of scientific treatises. One of these treatises, Aristotle’s On The Generation of Animals, takes up where his earlier book, The Parts of Animals, leaves off. Hence Aristotle starts the book by saying, “We have now discussed the other parts of animals,” whereas, he explains, the current book will focus on the generative parts of animals – that is, the genitals – and on how generation (or reproduction) occurs.

Aristotle’s On the Generation of Animals

Aristotle uses a lot of terms that are most likely unfamiliar to modern and unscientific audiences, and so a glossary of terms is provided below. You do not need to memorize these words and will not be tested on them! They are only provided to help you read Book 1 of On the Generation of Animals, which is the first assigned resource for this module.


  • Vivipara – viviparous animals are those in which the embryo develops inside the body (e.g. humans, cats, dogs) and the mother gives live birth
  • Omnipara – omniparous animals are those in which the embryo develops outside the body (e.g. by laying eggs)
  • Sanguinae – animals with red blood (with the exception of red-blooded worms, sanguinae correspond with vertebrates)
  • Cephalopoda – molluscs
  • Crustacea – crabs, lobsters, shrimp, barnacles, crabs – mainly marine animals though some (like hermit crabs) have adapted to life on land
  • Catamenia – menstrual blood
  • A priori and a posteriori arguments: a priori arguments are arguments based on rational deduction, or logical reasoning alone, whereas a posteriori arguments are based on experience or on empirical observations. We can think of a priori arguments as similar to mathematical proofs, while a posteriori arguments are more like biological observations.

The Four Causes

Aristotle also presupposes in this book an understanding of what he means by “causes” as these have been explained in earlier works of his. Aristotle distinguished between four different causes. None of these four causes is exactly like the modern sense of cause: we usually speak of the causes of events, whereas Aristotle usually speaks of the causes of substances. What is the cause of a rock, for instance, or of a star, a dandelion, or of your instructor’s cat Antonia?

Below is an account of Aristotle’s Four Causes. Again, you don’t need to memorize this information and you will not be tested on it. This explanation is just provided as background for the lecture and the reading.

  • The formal cause – what it means to be x (a rock, a star, a dandelion, or Antonia) – the definition, form or essence of x
  • The final cause – what x (a rock, a star, a dandelion, or Antonia) is for
  • The material cause – what x (a rock, a star, a dandelion, or Antonia) is made of
  • The efficient cause – what produces x (a rock, a star, a dandelion, or Antonia) or set it into motion. (This is the closest to our modern sense of cause as it is what we mean by cause when we speak of cause and effect.)

It helps to consider some examples.

First Example: A Table

  • Formal Cause – the form, idea or definition of a table, i.e. four legs, a flat surface
  • Final Cause – a perfect, completed table
  • Material Cause – the pieces of wood (or other material) that make up the table
  • Efficient Cause – the carpenter, and in particular the carpenter’s actualization of his idea of the table

Second Example: A Squirrel

  • Formal Cause – the form/essence of a squirrel, a squirrel soul
  • Final Cause – a mature, healthy squirrel
  • Material Cause – fur, bones, teeth, claws, blood
  • Efficient Cause – the generation of the squirrel or whatever the parent squirrels do that results in the actualization of the baby squirrel

The Four Causes Applied to Animals

Aristotle says he has dealt with the first three kinds of cause, with respect to animals, in other works. He is referring to his discussion in De Anima (On The Soul) of different living things (plants, non-human animals, humans), where he argues that the soul of an animal is its Formal Cause and, in a sense, is also the Final Cause. And then, in The Parts of Animals, Aristotle has discussed the material causes of animals, what kind of matter makes up their bodies. So, between these two books, he has already described the Formal, Final, and Material causes of animals. It is the last type of cause – the Efficient Cause – that Aristotle wants to explore in The Generation of Animals in relation to animals. In other words, his question is: What causes animals to come into existence in the sense of setting them into motion or instigating their development? We might rephrase this as: How does conception occur? How does life begin?

A carpenter’s hands implementing his idea

Aristotle understands the Efficient Cause in the case of things like tables: it is the actions of the carpenter that cause the table to come into being, and in particular it is the carpenter’s idea of the form of the table (or of the Formal Cause and Final Cause) which dictate what he does with his hands. So, ultimately, although the carpenter requires his hands to implement the idea, it is the carpenter’s idea which is the Efficient Cause of the table. But what is the Efficient Cause of a living being? It obviously has something to do with sex: animals have sex and somehow this instigates the existence of another animal. But how does this work? This is the topic of On The Generation of Animals, and is explored in the video lecture below.

Video Lecture: Aristotle’s On the Generation of Animals (48:35)

The Homunculus Theory of Conception

After presenting his argument for why sexual difference exists among animals, in the remainder of Book 1 of The Generation of Animals, Aristotle tries to refute competing theories of reproduction that were prominent at the time. One of these – the homunculus theory – gave the mother even less of a role to play in reproduction than Aristotle gives her.

A homunculus

According to this theory, the male semen has a miniscule baby in it already, complete with body and soul, that is too small for the eye to detect. The father inserts the baby into the mother via ejaculation in the way that a farmer plants a seed into the earth. This is quite similar to Aristotle’s theory except that the homunculus contain a tiny amount of matter, whereas Aristotle denies that the male contributes any matter to the child at all. The homunculus theory is arguably even more sexist than Aristotle’s explanation of reproduction because it denies that the mother contributes anything to the child other than the ground in which to grow. Nonetheless it was a fairly intuitive way of understanding conception since it was based on the metaphor of growing plants. Semen means seed, and the seeds of plants work this way, so it was easy to imagine that animals might come to life in the same way.

A seed sprouting

We saw the homunculus theory of conception in Aristophanes’ speech in the previous module, when he refers to the Circle People casting their semen onto the ground to reproduce. Thus, for Aristophanes, humans used to reproduce by planting their seeds in the ground just like plants do. After the Circle People have been punished by Zeus, however, the males has to plant his seeds in females. According to this theory, female bodies are the equivalent of the ground or earth.

A problem with both the homunculus theory and Aristotle’s theory, however, is that they do not explain how it is that a child might resemble both parents, and may even resemble the mother more than the father. If you plant a sunflower seed in one patch of ground or another patch of ground and it grows, it will look pretty much the same in either location: it may be more or less nourished and healthy depending on the patch of ground, but it is not going to grow up to be a rose or a turnip just because you plant it somewhere else. The seed determines everything. But in the case of animals, including humans, the same father can “plant” his semen in different mothers, and the offspring will look very different depending on the mother. Indeed, they may even look more like the mother than the father, or not look much like the father at all. It seems that the role of the mother is not just to make the child grow, but also contributes to the form of the child. The mother, in other words, seems to be contributing form and not just matter.

Side-by-side photos of a mother and daughter at the same age

Galen and the “Struggling Semens” Theory of Conception

Another theory of conception that was popular in ancient times was better equipped to explain this issue of resemblance. According to this theory, during conception the male and female semen struggle to dominate, and the female semen dominates as often as the male. If the female semen dominates, the child looks more like the mother, and if the male semen dominates, the child looks more like the father. Galen’s On Semen, which was written 500 years after Aristotle’s Generation of Animals, argues for this view of conception.


Galen was a Roman physician and medical researcher lived from 129-200 A.D. He was born in a Greek city that was part of the Roman Empire in what is now Turkey. Galen performed brain and eye surgeries that would not be attempted again for almost 2 millennia. He wrote over 600 treatises, most of which were lost when the library in Alexandria burned during the fall of the Roman Empire.

Analogous Bodies

With Galen we see an even more consistent view of male and female bodies as analogous than we saw in Aristotle. Aristotle, to recall, describes women as being like boys, impotent men, and mutilated men, so as less-than-perfect or less-than-fully-mature men. But Aristotle does note some differences between males and females, which Galen will not:

  • He thinks that males and females produce different kinds of reproductive fluids – women have menstrual blood whereas men have semen.
  • He recognizes that women can generate without feeling pleasure, which no one else seems to have recognized at the time.

Galen doesn’t allow for these differences. He understands women as almost identical with men, with the same reproductive organs (simply differently positioned) and with the same reproductive fluid that they simply discharge into their own bodies instead of into the other person’s body.

  • He refers to the ovaries as “testicles.”
  • He thinks women and men both produce generative semen which they emit during sex.
  • He thinks both women and men have to have orgasms for reproduction to occur.
  • He claims that semen builds up in “spermatic vessels” in the women just like it does in men.
  • He claims that women, like men, have nocturnal emissions or “wet dreams,” writing of “females experiencing effusions in sleep as males do,” particularly if they haven’t had sex in a while. He even describes a case of a woman who becomes hysterical because of a build-up of semen, who then she discharges the semen and claims that it felt pleasurable in the same way as sexual intercourse.

Galen’s Refutation of Aristotle

Ancient Greek depiction of two men boxing

Throughout most of On Semen, Galen is arguing against Aristotle: Book I is on male semen, and Book II is on female semen, and he disagrees with Aristotle in both books with respect to male and female anatomy and what he thinks the function of semen is in reproduction. In contrast, he approves of Hippocrates on these matters. He cites Hippocrates approvingly, for instance, for the following claim:

“If the semen from both remains in the uterus of the woman, first it is mixed together, since the woman is not quiet, and it collects and thickens as it is heated.”

Meg Ryan faking an orgasm in When Harry Met Sally

Galen thought that the female orgasm had three purposes:

  • The female orgasm causes the emission of female semen that is necessary for conception to occur.
  • The contractions of the uterus during the female orgasm serve to “grasp” the male semen and scoop it into the uterus.
  • The same contractions of the uterus then mix the male and female semens together into “one complete semen.”

Galen even cites male contemporaries who claimed that they could feel their semen being grasped by the female’s uterus during sex.

Ancient Greek sculpture of two men wrestling

Galen claims that Aristotle’s denial of the existence of female semen is simply a denial of obvious, visible evidence. He says that people like Aristotle who deny the existence of female semen are either “blind,” if they don’t see it, or “dull-witted,” if they see it but fail to enquire into what it is. Aristotle would fall into the latter category, for Galen, since he knows what people are referring to when they talk about female semen, but he denies that it is generative. He doesn’t give an explanation of what the substance in question is doing in the woman’s body, however, and this is strange since Aristotle claims that Nature does nothing in vain.

Family resemblance between a mother and daughter

Galen then raises the counter-argument to Aristotle’s theory that the male alone contributes semen, and therefore form or soul, that was mentioned above. He asks: if only males are contributing semen or soul, then why does the baby look as much like the mother as the father? For Galen it seems clear that since babies look equally like both parents (over all, not in every individual case), both parents must be contributing the same sort of product, and he thinks it is obvious that this is semen.

Finally, Galen also has another argument against Aristotle’s theory that the father alone contributes the soul or form to the baby: if this were true, Galen observes, when a female mare mated with a male ass, the offspring would be an ass. The offspring of a mare and an ass is, however, not an ass but a mule.

An ass and a horse

Ancient Views of Gender and Reproduction in Contemporary Film

If these ancient theories of conception seem very far removed from our own understanding of reproduction, consider the following representations of conception in popular, 20th-century American films.

Everything You Always Wanted to Know about Sex* (*But Were Afraid to Ask) (1972)

The sperm scene in Woody Allen’s Everything You Ever Wanted to Know About Sex* (*But Were Afraid to Ask)

In this scene we see sperm represented as individual little men with a purpose – they have separate personalities and intellects, and they know what they are doing. This scene is Aristotelian in so far as the future baby’s soul or mind is already fully present in the sperm, leaving little for the female to contribute other than matter. It is also a lot like the homunculus theory, where the mini-future baby is already entirely contained in the sperm. The film can be taken to reflect either Aristotle’s view of conception or the homunculus theory.

Look Who’s Talking (1989)

Opening Credits of the 1989 film Look Who’s Talking

The opening scene of this Hollywood film once again shows an Aristotelian version of conception. First you see the female contribution to reproduction, the egg, bobbing down – the egg is not anthropomorphized, it does not have a female voice, it is not thinking about what it is doing, it is not talking, it just seems to be floating like a bubble. It is a relatively big blob of amorphous matter and it drifts into place, rather than swimming there with a sense of purpose. In contrast, the sperm are like tadpoles that swim actively towards the egg. The sperm know what they are looking for and have a sense of purpose; they can talk to each other and encourage each other, knowing what it is that they are after. The one that will fertilize the egg has a voice, and we hear him speaking as he swims towards the egg and burrows into it, and his voice and personality are that of the future baby. So the baby’s character and voice, his soul or personality, already exist in the male sperm.

Kristie Alley and John Travolta in Look Who’s Talking

The credits say the mother’s name “Kristie Alley” as the camera focuses on this egg, and “Bruce Willis” (who will be the voice of the baby) as the camera focuses on this sperm. So the egg is female and the sperm are male. The sperm are tiny and white and active and they have almost no matter, whereas the egg is big and heavy, material and passive, simply waiting to be penetrated.

In reality, in the majority of cases the sperm gets to the place in the womb where conception takes place first, and waits there for up to 3 days before the egg arrives. The egg is likely to be the one to come and meet the sperm, rather than the sperm swimming to find the egg. This is because the egg is only there for 24 hours: if not fertilized in that time it gets flushed out, whereas the sperm can survive for up to 3 days. This means that it is statistically more likely that if the egg and the sperm are in the same place at the same time, the sperm were there first. Nonetheless in movie representations the egg is always already waiting to be penetrated, and the sperm swim towards it.

Ancient Views of Gender and Reproduction in Modern Science

We might think that while popular culture representations of reproduction are biased in the ways that ancient science was, projecting gender roles onto biological processes, contemporary scientific accounts of reproduction would be more accurate and objective. However, in “The Egg and the Sperm: How Science Has Constructed a Romance Based on Stereotypical Male-Female Roles,” feminist anthropologist of science Emily Martin demonstrates that this is not the case.

A depiction of an egg, sperm, and DNA against the backdrop of gender symbols

As Martin shows, in the late 20th-century context in which she was writing as much as in Aristotle’s day, social constructs of gender influenced the ways that biological processes were described. Indeed, reminiscent of Aristotle’s biological treatises, Martin finds that the role of the male in sexual reproduction continues to be elevated above that of the female, depicted as more active and admirable. For instance, Martin shows that 20th-century scientists described male ejaculation as a “remarkable” “feat” while female ovulation was simply “debris.” Sperm were cast as aggressors and decision makers, while eggs were described simply awaiting their fate.

“[Scientists] Gerald Schatten and Helen Schatten liken the egg’s role to that of Sleeping Beauty: ‘a dormant bride awaiting her mate’s magic kiss, which instills the spirit that brings her to life.’ Sperm, by contrast, have a ‘mission,’ which is to ‘move through the female genital tract in quest of the ovum.'”

Emily Martin, “The Egg and the Sperm”
Sleeping Beauty

In fact, however, Martin shows that such representations of reproduction are inaccurate, since eggs are at least as actively involved in the process of conception as sperm, even trapping and enveloping the male gametes. As Martin shows, however, in the rare scientific accounts of conception that acknowledge the active role of the egg, the egg and sperm continue to be gendered. In some cases, the activity of the egg is cast as similar to the activity of a woman in a traditional courtship.

“The egg selects an appropriate mate, prepares him for fusion, and then protects the resulting offspring from harm. This is courtship and mating behavior as seen through the eyes of a sociobiologist: woman as the hard-to-get prize, who, following union with the chosen one, becomes woman as servant and mother.”

Emily Martin, “The Egg and the Sperm”
A depiction of traditional courtship.

In other cases, however, scientific depictions of reproduction that acknowledge that active role of the egg resort to spidery images and stereotypes of women as dangerous to men.

Spider Woman

Importantly, Martin argues that the feedback loop between the social and the scientific moves both ways: not only do socially constructed ideas about gender impact the ways that scientists talk about reproduction, but the ways that scientists talk about reproduction contribute to and reinforce the social construction of gender. “The Egg and the Sperm” is the second assigned reading for this module, and in this article you will see that Martin provides not only a plethora of examples of gendered representations of male and female biological processes and anatomy, but also proposes less sexist ways in which we could talk about reproduction.