An idea forms when the mind recognizes or interprets an impression, or connects two or more impressions.
A single impression may be felt, remembered, or noticed, but it becomes an idea when the mind recognizes something about it or relates it to something else: cause, category, meaning, judgment, pattern, or possibility.
In chapter 18 of 30 Philosophers, I define an idea like this:
“An Idea is a mental construct that stems from impressions, describing concrete objects or abstract entities of the material world or beyond.”
Ideas are the thoughts we use to interpret life. They include words and stories. They involve categories, beliefs, and values. They can be a model, a theory, or a framework. An idea can describe a rock, define justice, imagine heaven, explain gravity, or ask what it means to live well.
In TST philosophy, ideas are powerful, but they are not the same thing as reality. The word “tree” is not a tree. A map is not the land. A theory of gravity is not gravity itself. Ideas help us navigate the material world, but they can also mislead us when we mistake them for the world.
This is why TST pays so much attention to sorting ideas. Empirical ideas describe the material world directly, rational indirectly, and irrational not at all. Ideas are the second side of the foundational TST split. The material world is reality. Ideas are our descriptions of it.