The division of the objective and subjective is not as clear as as the easy use of the terms would suggest. The terms are adjectives, after all, but its not entirely clear what they modify. The subjective and the objective don’t appear to be two different kinds of knowledge, since objective knowledge seems redundant and subjective knowledge impossible. They are not two different kinds of belief for the opposite reason. Are they two kinds of “mental acts” perhaps? On this account, since a mental act is clearly of a subject, then to call it “objective” would be an addition to its subjectivity. Objectivity then becomes subjectivity plus something else, that is, objectivity becomes a kind of subjectivity, the way horses are kinds of animals. But so defined, the very opposition between the objective and subjective vanishes, which is exactly what we are trying to explain.
Nominally, the objective is what is “of the object” and the subjective is what is “of the subject”. The distinction arises from some difficulty in analyzing sensation. I walk in a room and think its warm, you walk in and think it’s cold. So is the room warm or cold? The difference depends on the subject, we say. It is not a feature of the object. Viola, subjective knowledge.
But is it as easy as all that? Both of us are making statements about the room, after all, and specifically about its temperature. Differences in judgment don’t suffice to say knowledge is subjective anyway: if you think the room is terribly cold and I think it’s fine, but you have a fever, then we both think that the room is actually fine. Likewise, if some human being didn’t think 140 degrees F was hot, or -40 degrees F was cold, this would be a failure to understand their environment well. This seems most of all where the subjective arises. The polar bear wouldn’t find -40F too cold. Some temperature fatal to us is healthy for polar bears, so temperature is subjective. On this account, “subjective” is brought into explain a discernment of harmfulness. The thing is not harmful in itself, but in relation to us: as Augustine would say, if poison is destructive in itself, it would kill the snake first. And so it seems what we mean by “subjective” is good or evil for a given nature or subject. Similar considerations apply to taste, and in some way to the sense of smell. Presumably, dung beetles are attracted to the smell of poo, and even if they weren’t, it would be an advantage to them to like the scent.
Subjective vision is something different. Deer don’t see a difference between orange and green. Why do we have to invoke the subjective to explain this? Why not simply say that we see things deer can’t?
If this is right, than the division between the objective and subjective is not between two kinds of knowledge or belief or mental act, but between knowledge as of the known, and as of the known so far as it befits the nature (the right temperature, pleasant scent, appealing taste). The first are knowledge as objective, the second as subjective. So considered, subjectivity is more in the order to appetite than knowledge, and so would be secondary to objectivity.