In philosophy, ideas are usually construed as mental representational images of some object. Ideas can also be abstract concepts that do not present as mental images. Many philosophers have considered ideas to be a fundamental ontological category of being. The capacity to create and understand the meaning of ideas is considered to be an essential and defining feature of human beings. In a popular sense, an idea arises in a reflexive, spontaneous manner, even without thinking or serious reflection, for example, when we talk about the idea of a person or a place.

One view on the nature of ideas is that there exist some ideas (called innate ideas) which are so general and abstract that they could not have arisen as a representation of any object of our perception, but rather were in some sense always present. These are distinguished from adventitious ideas which are images or concepts which are accompanied by the judgment that they are caused or occasioned by an external object.