How do we inherit properties from our parents, according to Aristotle?

Ancient philosophers tried to explain how children inherit properties from their parents and ancestors. In the Generation of Animals, Aristotle (384 - 322 BC) presented one of antiquity’s most sophisticated answers.

While Aristotle was certainly not alone in holding forth on this question, some of his predecessors, such as Plato (428 - 348 BC), Aristotle’s own teacher, were surprisingly reticent. In the Timaeus, for instance, Plato says that the man’s seminal fluid contains living things that are nourished in the woman’s womb before they eventually see the light of day. No doubt Plato has interesting and compelling thoughts about embryology — but where is the answer to the burning question: why do kids look like their parents, and, for that matter, their other ancestors, too?

Aristotle’s determination to answer this question is one of many reasons why the Generation of Animals is such a great and important contribution to the history of ideas.

The food that we eat is concocted and transformed into blood. We use our own natural heat to effect this concoction, and some of the blood is further concocted into semen. That’s what happens in men. Women are naturally colder than men, and so while they have enough heat to turn food into blood, they can’t turn blood into semen. The result: women have an excess of blood, some of which is discharged as menses.

Aristotle thinks that the man’s semen contains the form of the human being. This means that the semen can initiate a transformation that results in something that is designed as a human being. It can make actual the potential that something else has to be a human being.

In Aristotle’s terminology: semen can actualize something’s potentiality to be human.

What has that potentiality? The woman’s menses.

Women don’t have the heat required to make humans on their own, but their menses provides the material, or “matter” as Aristotle would put it, of the offspring. But since the menses has merely the matter — the stuff that becomes the body of the offspring — what the menses needs is the semen, which was concocted well enough to contain the form of the human being.

Think about it like this. The menses is the bricks that are needed to build a house. But until the bricks are shaped like a house, they are merely a potential house. What the bricks need is some kind of blueprint, design, shape, or form to transform them into a house. The semen delivers that design and initiates the relevant physiological transformations.

So, that’s how babies are made. But why does one baby look like the father, whereas another looks like their mother? And why do we sometimes notice, say, hair color of neither the father nor the mother but of a grandparent, instead?

That’s because of what the semen does when it actualizes the menses’ potentiality, and what it tries to do.

Semen actualizes the menses’ potentiality by trying to “master” it. The semen is aiming at making the offspring look like the father. That’s part of what it means for it to be imparting a “form,” or shape, onto the menses. It has to overcome or master the menses’ qualities.

In the ideal case, the semen will make the offspring look exactly like the father. That explains why some children have the same qualities as their fathers. But it basically never happens that the child looks identical to the father, and sometimes children even turn out to be female! What’s going on?

Well, in that case, the semen wasn’t perfectly concocted. The less perfectly concocted the semen is, the less capable it is of mastering the menses. In that case, the offspring sometimes turns out to be female, or, more mildly, the child turns out to be a male with some characteristics of the mother.

Here’s a crucial passage from the Generation of Animals:

“If the spermatic residue in the menses is well concocted, the movement from the male will produce a shape [or form] that corresponds to itself. For it makes no difference whether we say the semen or the movement that makes each of the parts grow, or again what makes them grow or what composes them at the start (for the account of the movement is the same [in either case]). So if this movement masters, it will produce a male, not a female, and one that is like the male progenitor and not like the mother. But if it does not master, with regard to each capacity, whatever it is, that has not mastered, it makes [the embryo] correspondingly deficient” (GA IV.3 767b15-23).

This long passage says much about Aristotle’s view.

If the semen is concocted well, the result is a male that looks like the father. But if the semen does not master, then the embryo becomes deficient when it comes to whatever respect it did not successfully master.

Why does this happen? Well, the explanations are always going to mention some shortcoming of the man’s natural heat, which in turn leads to the imperfect concoction of the semen. For instance, perhaps the man is too young or too old; in such cases, the natural heat isn’t as great as it would be in the man’s prime. Aristotle thinks that men outside of their prime are more likely to have daughters.

This passage is the crucial one:

“Now since everything is shifted not into any random thing but into its opposite, and what in generation is not mastered is necessarily shifted with regard to the capacity in which the progenitor and mover has failed to master, and becomes its opposite. If, then, it has not mastered insofar as it is male, it becomes female, if insofar as it is Coriscus or Socrates, it becomes not like its father but like its mother. For as what is opposed in general to the father is the mother, what is opposed to a particular male progenitor is a particular female progenitor. And likewise with respect to the capacities that stand next in order. For it always changes more to the ancestors, whether to those of the father or to those of the mother” (GA IV.3 768a2-11).

The first sentence tells us that when the semen fails to master, the result is not random. It is, precisely, the opposite of what the semen is trying to achieve. For instance, the semen is trying to make the offspring male like its father. If it fails, the offspring becomes the opposite: female.

The result of the passage gives us examples. Even more interesting is the mention of the ancestors: Aristotle means that if we imagine a “battle” during which the semen tries to master the menses, sometimes things can revert back to qualities inherited from ancestors. In this way, the inability of the semen to properly make the impression that it wants can lead to ancestors’ qualities making their impression.

So, things aren’t as simple as just the man’s qualities winning out, or the woman’s. The efforts of the man’s semen can sometimes lead to the resurfacing of qualities from previous generations.

In conclusion, this is the outline of Aristotle’s sophisticated and robust attempt to answer an age-old question: how do children inherit qualities from their parents and other ancestors?

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