INTPs use Si as their third function, while ISTJs use it as their dominant function. Trying to conceptualize Si is a little difficult, but at least not as difficult as Ni.
While Se is objective, Si is subjective. With Se, the object of attention is an end in itself. Si, however, deals with how you senses relate what you see to your life experiences.
Si is the opposite of Ne. While Ne is always looking at future possibilities, Si tends to be locked in the past. As a result, Si is very traditional. So, when you touch a Nintendo, and get memories of playing Super Mario Bros., that is Introverted Sensing. Si also senses the atmosphere of a place.
"The urge to dream, and the will to enable it is fundamental to being human and have coincided with what it is to be American." -- Neil deGrasse Tyson
intp '82er