Long before rationalists and empiricists argued about how to know, we wondered what can be known. If we allow for even the most unreasonable doubt, what is still certain? In this rigid framework, one truth is not conceivable as an illusion:

Thoughts exist.

Aristotle said, “to be conscious that we are thinking is to be conscious that we exist”.

When Descartes, often labeled the “Father of Rationalism”, arranged this as, “I think, therefore I am”, some say he assumed too much. One can argue that the undeniable in “I think, therefore I am” comes from the empirical – a unique hypothesis that, when made, provides observable evidence confirming it:

Thoughts exist.