I have been researching a lot about medical theory during the early modern/renaissance period (16th-17th century) and found some really interesting things. I don't know how much, if any, of this research will ultimately end up in a fiction piece that I am doing. So I wanted to share because I think it's really fascinating stuff. One of the books I've spent a lot of time with is Making Sex: Body and Gender from the Greeks to Freud by University of California Berkeley's Thomas Laqueur. Most gender historians seem to focus on gender and its social history, Laqueur reminds us that there is a social history of sex, too (when I say "sex" I am referring to it in the biologically understood male or female sense and not the intercourse sense), and that ideas of sex have been shifted by the social/cultural milieu just as much as ideas of gender. One chapter called "New Science, One Flesh." explains the medieval/early modern medical conceptualization of sex.
Whereas today we GENERALLY understand that there are two distinct sexes (I'm aware there are exceptions), either male or female, this was not how early modern academic understanding conceived it. Biologically determined sex (as opposed to socially determined gender) was the mutable factor whereas socially determined gender was the fixed, divinely ordained factor. Interesting we generally understand this the other way around today, I think. You are born a particular sex, but gender identity is flexible. It seems almost the opposite in early modern thinking.
In the 16th and 17th century, and well before that, your sex was determined by the amount of heat present at your conception. A hot enough conception would yield a male. A conception lacking enough heat would yield a female, the less desirable outcome. This was understood on a continuum, though, in the context of ONE sex (a masculine one, of course), and NOT two distinct sexes. Thus a woman's anatomy was described completely in terms of being inferior or colder VERSIONS of a man's anatomy. A vagina was an inverted penis, and the ovaries were undescended testes. Indeed, the same terminology was applied to what were thought to be the same organs in men and women, instead of testes and ovaries, you just had testes, external if male, internal if female. Instead of scrotum and womb, you had one word "bourse" or "bursa" which meant sack or purse, external if male, internal if female. Medical thinkers of the age called these organs the same thing because they theorized them AS the same thing, even though the organs served completely different functions (when, guys, was the last time you found a fetus in your... I'll stop there). Had there been enough heat during the woman's conception, these organs would have simply "sprouted" out of her and she would have been a man.
Thus, a man that lactated was thought to be "colder" than the average man, and a woman that had more masculine physical features would be thought of as a "warmer" woman. And indeed, this theory also accounted for people born with both female and male genitalia, someone who, temperature-wise, was caught in the middle. So there was this weird continuum when it came to your biological sex that could even be shifted AFTER you were born, according to medical understanding.
For example, the theory of the four humors, an idiosyncratic balance of hot and cold, dry and wet internal body substances that determined a persons health, necessarily implied that the balance of hot and cold could be manipulated (through blood letting or ingesting hotter or colder liquids) to make someone more masculine or more feminine if that individual was seen to be either physically or socially defective. This was just one part of a health system that could account for ANY physical or mental ailment, not just sexual. A headache or a quick temper could mean that one had an excess of hot blood, which needed to be drained. I find this idea of an understood continuum of biological sex really fascinating. Even though the ability to change one's biological sex surgically has only come about very recently, these issues of "what is sex?" and "can sex change?" are more than 500 years old.
The other interesting thing is that, despite the new science of exploratory dissection becoming much more ubiquitous during this time period (16th-17th century) due to the work of physicians like Andreas Vesalius, which greatly shifted understandings about anatomy and health and disease, the understanding of female anatomy remained basically unchanged for a few more centuries. Anatomical drawings from the period are fairly accurate (see Andreas Vesalius' De Humani Corporis Fabrica from 1543), so we know that these things were observed (again, these people never saw a fetus inside a scrotum even though they theorized it to be the external correspondent to the womb), but the social interpretation of them remained fixed. This reminds me of mischaracterizations of conception that still exist today: the passive egg, the charging sperm! Even though that is not an accurate reflection of what actually goes on during conception, that description is still informed by cultural understandings of men and women. I think this is also a good case study of how science is not immune from the social forces at work in the surrounding culture, yes, even today.
How will this all end up in my fiction you ask? I'm not sure. I think one of the great opportunities that writers of historical fiction often miss is the ability to present a consciousness that is completely different from our own. The idea that a medical scholar in 16th century London walked down the street and understood the men and women that passed him by in a COMPLETELY different way than I do is literary gold! Aristotle and Plato had different, in some cases, completely opposite views of the nature of reality itself. How did this change how they physically looked AT things, how they saw themselves in the world. If as a writer, I can briefly get someone to look at the world through these different lenses, that is a very rich experience to offer.
There are a few books I can think of that create a completely new consciousness pretty well. Umberto Eco's The Name of the Rose is one of the best historical examples I can think of. Non-historical examples include Denis Johnson's Jesus' Son and Marilyn Robinson's Housekeeping. All of these books present ways of looking at the world that are unfamiliar and new. Every sentence is imbued with the shifted sense of reality that these authors have created. The thing that is particularly impressive about Jesus' Son and Housekeeping is that they DON'T take place in a different world, but contemporary Iowa and Montana, respectively. However, they MIGHT AS WELL be in a different world. This, again, is the tragedy of a lot of historical fiction. It DOES take place in a different world, but the craft, language, and vision don't push those boundaries to create a new consciousness and a new reality that bend the lens through which we look at OUR world.