This is such an important word in the philosophical vocabulary that I have to repeat here some of the things in the entry on Cognition.
In the ancient world it was Plato not Aristotle, who used the word "ideas" to signify intelligible objects of the understanding. He was correct in regarding ideas as intelligible objects, but incorrect in asserting that ideas exist in reality in addition to existing for the human mind as objects of conceptual thought,.
On this point, Aristotle corrected Plato's error by using the word "concept" for the mental content that intend or signifies ideas as intentional objects. It was Aristotle and, after him. Aquinas who explicitly distinguished between that which the intellect understands and that by which it achieves such understanding.
This distinction between the id quod (that which) and the id quo (that by which) of our intellectual acts prevents us from ever saying that our concepts are that which we are conscious or aware of when we understand ideas. We could not be aware of the concepts in our minds and also at the same time be aware of their intelligible objects. If we were, we could not distinguish between them. Which would mean we could not affirm that such objects exist and are shared by other minds.
This Aristotelian and Thomistic distinction between the id quod and the id quo of our intellectual acts of understanding has been completely lost in the modern world, beginning with Thomas Hobbes and Descartes, and especially with the nominalism of Bishop George Berkeley, David Hume, and John Locke.
The tradition of British empirical psychology and also of German and French psychology, used the word "ideas" for what were not ideas at all. In that modern tradition down to the present day, the word "idea" signifies the sensory content of the human mind -- its sensations, perceptions, memories, and images. All this sensory content was treated as that which we are conscious of when we are engaged in thought.
This understanding raised the insoluble problems of how there could possibly be real existence as objects of thought. A vain attempt to solve this problem consisted of regarding the sensory contents in our minds as representations of real existence.
If they cannot be so regarded, the next step is complete skepticism. If the sensory contents of our minds, our so-called ideas, could not be regarded as representations of reality, then we could have no contact with a reality that is independent of our minds.
Kant, failing to correct Hume's errors, affirmed real existence, things in themselves, but also, asserted that they are unknowable by us. From that time on in the modern world, the greatest of all modern philosophical mistakes was generated -- the error of idealism, denying that there is any knowable reality independent of our mind. No ancient or medieval philosopher was an idealist in this sense of the word.
Finally, Berkeley and Hume were nominalists. They denied the existence of what they called "abstract" ideas." All the common nouns in the vocabulary of everyday speech have general significance, referring to thing s that are the same in kind. Berkeley and Hume tried to explain how this was possible in the absence of any intellectual content in the human mind -- in the absence of any acts of conceptual thought and of abstract ideas as intelligible objects.
Adler's Philosophical Dictionary (1995)
Great Ideas from the Great Books (1963)
by Mortimer J. Adler