He set apart three kinds of ideas, each of which can exist without the intervention of experience. Those three kinds of ideas are called fabricated ideas, adventitious ideas, and innate ideas.

The first idea, the fabricated one, is simply an invention of our mind. What makes this idea different from the other two is its characteristic which can be put aside and altered anytime if we want to. Drafts of fiction story plans for tomorrow, and sketches of modern art, as long as they are still in our mind, are some straightforward examples of this fabricated idea. They can be set aside anytime, and they can also be modified at will.

In contrast, an adventitious idea is a perfectly firm idea in our mind that cannot be set aside or changed just because we want to. This adventitious idea is basically a kind of sensation in our mind which is caused by solid external reasons. These external reasons can be anything, like fire for example. If we stay close to the fire, we obviously cannot set aside the sensation of heat in our mind, nor can we change it to another sensation simply by wishing so.

The last one, the innate idea, is the idea which God has inscribed in our mind since birth. It is possible to set aside the idea and observe it, but it is impossible to change the original characteristic and concept of that idea. The ideas of mathematical equations, geometrical shapes, and time concepts are some of the popular examples of innate ideas. We can easily set aside and observe the idea that one plus one equals two, that triangle has three sides, or that what is done cannot be undone. However, no matter what we do, we definitely cannot change the essence of those ideas.

Descartes’ concepts of ideas, especially the innate one, unquestionably support the theory of rationalism. The concept that God provides us inborn ideas means that we have been armed with knowledge since birth. Thus, it can be concluded that experience is not always required to gain knowledge, which ultimately makes this idea a branch of the basic principle of rationalism theory.