Because rationalism is a method, it does not answer the question of where knowledge comes from. All rationalists thus accept one or more of the four views about the origin of ideas. Yet historically, the rationalist method has been strongly associated with the idea of innatism. The terms "rationalist" and "rationalism" have thus often been used to describe those who accept both the rationalist method and innatism. To avoid confusion, those who accept the rationalist method but do not necessarily accept innatism we will call rationalists while those who accept both the rationalist method and innatism, we will call innatists. Innatism (sometimes called "Narrow-sense" rationalism) is thus included within the term rationalism.

Innate knowledge is inborn knowledge which is universal, 100% certain, and logical. Innate knowledge is already within a person at birth. There are several explanations as to how it got there. Some who say we have a previous life while others would say God put the knowledge in us at birth. Innate knowledge is universal. That is, it is inborn in everyone everywhere (though they may not be able to access it). Innate knowledge is also 100% certain--it has to be correct.

Innatists might use the idea "equals added to equals yields equals" as an example of innate knowledge Such an idea is innate, they argue, because it is something that we do not have to prove with experiments, but rather it is a statement the truth of which is immediately apparent to us. "A straight line is the shortest distance between two points" is another example of an innate idea, because it is immediately apparent to us. A second category of innate ideas is moral truths that are immediately apparent to all persons. "It is wrong to torture innocent children" is an example of a truth that Innatists argue is innate. But some rationalists argue that small children and sociopaths do not know that it is wrong to torture innocent children, so therefore the idea cannot be innate.