What does Thomas mean by subject? "Object" seems to be the thing that is potentially knowable. "Medium" Thomas divides into knowing by "demonstrative" or "probable" mediums. By "subject" he appears to mean the knower.
Moving on, Thomas says this:
On the part of the subject the difference of perfect and imperfect knowledge applies to opinion, faith and science. For it is essential to opinion that we assent to one of two opposite assertions with fear of the other, so that our adhesion is not firm: to science it is essential to have firm adhesion with intellectual vision, for science possesses certitude which results from the understanding of principles: while faith holds a middle place, for it surpasses opinion in so far as its adhesion is firm, but falls short of science in so far as it lacks vision.What he says of opinion is interesting, and psychologically astute in the sense that "fear of the other" is sometimes a key feature of our opinions. That said, it doesn't hold up. Would we say, for example, that old theories about continents being stable were just opinions and that the global plate tectonics theory that is now dominant is not? Once upon a time, most scientists assented to the first and now most (meaning virtually all) assent to global plate tectonics. The temptation is bang the table and say, "We know that global plate tectonics is right!" The problem with saying that, though, is that as recently as the 1950s most scientists would have banged the table and said they knew it was wrong.
We must grant that Thomas means something different when he uses the word science than we do but his definition is broader rather than narrower so the objection applies. He did not have the historic awareness of physical science that we have. He was, of course, aware that mistakes could be made but the notion of a change of paradigms was not available to him. To charge him with a mistake in this regard would be like criticizing Hannibal for using elephants instead of tanks against the Romans. That said, we do have a greater historical awareness and we cannot make the sort of distinctions Thomas makes between opinion and science above.
For us, what is admirable about science is the lack of adhesion. It is precisely because science is always open to challenge that we are willing to assent. Some opinions, on the other hand, are not falsifiable. Someone can call my opinion that Verdi is greater than Wagner stupid, they can assert that it is contrary to critical consensus but they can't prove it wrong. A musicologist with such an opinion could get hired by a university and, while others might decry this, it would be a very different matter if a physicist who did not believe in gravity was hired by a university.
Consider this example: every morning I get out of my bed and walk across the floor. I don't know that the floor won't collapse. Indeed, I know that floors sometimes do collapse. I don't worry about it though. You might say that I have a firm faith that it will not happen. I don't need to go downstairs and check that the floor joists haven't rotted although I know that can happen.
The same is true if you ask me if I know that evolution is right. I will say that I do because it seems to me that that particular foundation will fall out from under me.
I have beliefs and beliefs have consequences. I tend to call my beliefs "knowledge" when I am more certain about them and opinion when I am less so. Faith is what I assent to firmly, with adhesion, even when I have little grounds to claim certainty. And that, it seems to me, is all we can say.