I am logging what I witnessed in different fields, the example and potential contributing factors for two - three crevice. Exploring the intractability boundary, I theorized two is guarded by dual whereas three is attacked by the absence of consistency certificate. Bilinearity (observed in (X/M)\* = X/M\*) may be how dual contribute to consistency. - F = ma is second order - matrix has two sides leading to all "dual" theorems such as f\*\* = f - 1, i two complex analysis basis - “Two is a company, three is a crowd” (Churchill) - Three body problem does not have closed form solution - Heat equation is second order differential equation which is associated with brownian motion - a basis for a important class of stochastic process - Two matroid intersection is easy (polynomial solvable but near P-NP boundary as it is hardest within the easy) whereas three matroid intersection problem is Np-hard. So is 3 vs 2 satisfiability (note 2 Max-SAT is in NPC). - Normal distribution is the distribution with the highest entropy given two moment information (mean and variance) which stems from its self duality (same form of pdf and characteristic function). - Oxygen forming two, not three, degree bond (some other sigma and phi bond in orbital examples to be added)