“Generalization” and “Discrimination” are two complementary tools for thinking about the world. We generalize apples and oranges as “food”. We discriminate round things that are edible (apples) from what is not edible (rocks).
We generalize mathematical concepts, like quantities. Two houses and two apples both have “twoness”. One might imagine that two children who come upon two apples may behave differently than two children who come upon one apple. Concepts help us to deal with real problems in a real world.
Rational thought begins with real world experiences. We generalize the lessons learned from one experience by creating a model in our imagination. That model guides our subsequent behavior the next time we have a similar experience. Errors in judgment cause new problems that force us to rethink the problem, adjust our mental model, and re-evaluate our presumptions to get better results the next time.
The term “a priori” refers to the use of definitions to draw conclusions. We know that this tasty red fruit in my hand is an “apple” because we have agreed upon this name to refer to these objects. Given an orange or a strawberry, we can conclude from their definitions that they are not apples. And that is all that it means to prove something “a priori”. Immanuel Kant speaks of deriving moral laws by “a priori” or “pure” reason, independent of experience. But any concept that is so pure as to be independent of the empirical world is also likely to be meaningless and irrelevant.
All useful concepts, like “love”, “fairness”, “gravity”, “shape” and so on, are derived by generalizing something we have observed or experienced in the real world.
Other definitions may be less clear, like, what is “good” or “right” or “faith” or “love”? Yet we each find ourselves communicating more or less effectively using these terms in common speech.
Sometimes meaning will vary significantly with the context. Our “faith” in the basic goodness of those we love is somewhat different from “The Catholic Faith”. In the latter case, the word “Faith” might be replaced with the word “Religion”. What is actually meant may be different in the two contexts. Using a meaning from one context in a very different context can cause confusion, and tragic or comedic results. For example, your faith in a friend should not imply he is an object of worship.
So conceptions are generalizations from experience. And their specific meaning may vary according to the context (model) in which they are used. Therefore we also need the faculty of discrimination to handle the details of meaning in different contexts.