Concepts are defined as abstract ideas. They are understood to be the fundamental building blocks of the concept behind principles, thoughts and beliefs. In contemporary philosophy, there are at least three prevailing ways to understand what a concept is:

• Concepts as mental representations, where concepts are entities that exist in the mind (mental objects)
• Concepts as abilities, where concepts are abilities peculiar to cognitive agents (mental states)
• Concepts as Fregean senses, where concepts are abstract objects, as opposed to mental objects and mental states