The example is taken from a book (a peasant):
The life of a peasant in the Middle Ages was hard.
Why not the peasant?
The life of the peasant in the Middle Ages was hard.
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Of course in most contexts we use a to refer to a generic, non-specific example of some class (or the first mention of a specific member of that class), and the to refer to a specific previously-mentioned member of that class. This is not the case here.
In this usage, both a and the can be used to describe an exemplary peasant, taking some random peasant in the middle ages and using the quality of their specific life as a proxy for the quality-of-life for peasants in general. But they do not have the same connotation or usage in modern English!
Using a peasant is more humanizing. It is sort of like the second usage of a that I mentioned above—saying "If you look at a random peasant in the middle ages, odds are good that their life was hard." It allows that, of course, some peasants were relatively well-off and some were not; after all, everyone is an individual, and everyone has their own challenges in life.
Using the peasant is less humanizing; it is an abstract and detached view of "peasants as a rule" and says that you can know everything there is to know about all peasants just by studying a single example. See, for example, this webpage describing "The Life Cycle of the Fly" and note how, throughout the page, it refers to "the fly," "the egg," "the Drosophila female," etc. It is not polite to use this construction to refer to sentient humans, as it reduces an individual human (with self-determination and agency) to a mindless member of a general class. You might make an argument that it is more appropriate to apply this usage to lower-class workers, especially in feudal times, but it is not polite in any case.
In both cases, you’re using ’peasant’ as a generic, but the generic forms are different in their meaning.
A generic formed with the definite article in English is generally categorical. In other words, it talks about the noun as a category of person/place/thing/idea, describing all items referenced by that noun as one group. This is essentially the same as using a plural form as a generic, or alternatively saying ‘all X’ where ‘X’ is the noun in question.
A generic formed with the indefinite article in English is generally an ‘unspecified’ generic. It usually does not refer to all possible items referenced by the noun, but it often does not refer to a single instance either. This is essentially the same as saying ‘some X’ where ‘X’ is the noun in question, or occasionally ‘any X’ in some contexts.
Where this gets complicated is that in some cases, a generic noun with an indefinite article is still a categorical generic. This type of construct is normally used when talking about people, because it’s felt to be more ‘personal’ than using the definite article.
Given this, I would normally prefer the form with ‘a peasant’ over ‘the peasant’, but either is grammatically correct.
Both constructs are acceptable. However, there can be a difference in interpretation.
'A peasant' uses peasant as a generic category: that class of people known as peasants. 'The peasant' may also use peasant in this generic, categorical sense. However, 'the peasant' might also refer to a particular, singular person: 'The Peasant, name unknown, social caste - lowest of the low'.
In the singular case, the application of Peasant as a title is insulting and demeaning.
Of course, the singular application is not applicable in the example. However, I think that, amongst English speakers as a group, the knowledge that it MIGHT be applicable, even if that knowledge is subconscious, impacts the interpretation of the original message. Thus, "a peasant" is less likely to be felt to show bias or prejudice, than "the peasant". The difference, though, is slight.
What my English teacher told me, and it was a very simple to understand rule:
The first time you mention some object, its generic, unknown and unspecified and therefore you say "a(n) object". Later on in the conversation, if you keep referring to exactly that same object, its clear, known and specific which object it is so you say "the object".
I bought A CAR. THE CAR is red. THE CAR can go 200km/h.
If you would later on add "A CAR I bought can go 200km/h" you can't be sure is it about the red car from the previous sentences or are we now talking about a completely different, yet unspecified car.
We often use a when we mention something for the first time, and then change to the when it is clear which thing we are talking about: in the example it could be the first time that the peasant was mentioned, that is why we use "A" We also use the when it is obvious which thing we are talking about or when there is only one of something.