The choicest or most essential or most vital part of some idea or experience
Center
Centre
Core
Essence
Gist
Heart
Heart and soul
Inwardness
Kernel
Marrow
Meat
Nub
Pith
Substance
Sum
Cognitive content
Content
Mental object
Quiddity
Quintessence
Bare bones
Stuff
Haecceity
Hypostasis