Epistemologically, the first stimulus (my first stimulus) is sensory. It is of the things in the world, including the reflexive sensing of myself as a thing. The evidence is, things are known by sensation, and also things remain regardless whether being sensed or perceived.
You cannot perceive what is not first sensed. And you cannot conceive what is not first perceived. No chain of conception can occur without the first stimulus.
The chain of conception is the illusion of a self. The self is a conception.