Through our senses we encounter many individual things, gaining an intellectual knowledge of each of them, an idea in our minds which of course is not physical (except inasmuch as our physical brain neurons are involved) but is an idea, a mental conception of something. The way our ideas of objective things correspond to those things as they (if they) exist in the real world is an interesting question, but the problem of universals primarily regards the next level of consideration.
Logically subsequent to our experience and conceptualization of these individual things, we categorize them into different kinds of things and form universal concepts regarding those kinds of things. For example, we encounter some things like these:
google.com/search?hl=en&rlz=1G1TSNA_ENUS371&=&q=cat&gs_sm=e&gs_upl=319l776l0l964l3l2l0l0l0l0l414l553l0.1.4-1l2l0&um=1&biw=1366&bih=596&ie=UTF-8&tbm=isch&source=og&sa=N&tab=wi
And, from these, eventually develop an idea of “cat.”
Similarly but more profoundly, we may experience many beautiful things in this world, and eventually come to an idea of beauty, or experience many good things, and come to an idea of goodness.
All of these ideas are universal ideas, and the problem of universals asks whether such a thing as “cat” or “beauty” or “goodness” exists in the real world outside our minds.
Nominalists would say no, that “cat” for example is merely a name we apply to many diverse things out of a subjective need to categorize objects in an otherwise bewilderingly complex world. In our minds we may accompany this word with a vague image created from combining remembered images of individual creatures we have come across which we now wish to label as cats, but this vague image doesn’t correspond to anything real; only a remembered image of a real cat you once saw corresponds to something real.
Conceptualists, meanwhile, say more or less that universal ideas do exist, but we cannot know if or how they relate to the world of the senses, that we know them only as ideas we have in our minds and it is in that “ideal” way, that “conceptual” way that they have value.
Realists meanwhile affirm that universal ideas do correspond to something in the objective world outside of our minds (they think universals are “real”), but there are two very different camps concerning this.
Platonic Realism, or Exaggerated Realism as it’s sometimes called, says that there really are universal beings out there, which human reason can gain knowledge of. Generally the idea is that the individual instances of something our senses reveal to us, like this particular cat or that other cat, are somehow less real manifestations of a truly real Universal Cat, from which all individual cats we encounter with our senses are somehow derived.
Moderate Realism says that there is no such Universal Cat floating in some esoteric plain of reality, but that the idea of “Cat” really does apply to something in the real world, namely everything that can accurately be called a cat. Whatever traits are shared by each individual cat which makes them definable as cats, and without which a creature would be something other than a cat, that whatever-it-is defines a sort of feline nature. Our universal idea of “Cat” accurately corresponds to each and every animal with that feline nature, while things that set each cat apart from another are by definition merely “accidental” attributes that do not effect the thing’s feline nature. This position is distinguished from Nominalism in that nominalists deny the very universal idea of “Cat” as anything other than a label to apply to whatever we want to apply it to and a vague image created by mixing memories of different things we wish to apply that label to and corresponding to nothing real, while moderate realists say we really do develop a universal idea of “Cat” and this idea really does accurately describe something in the real world, not a single real Universal Cat but each real individual cat out there.
