&gt; So this must there is some characteristic of the problems that we generate in practice that makes them &quot;easy&quot; but we don&#x27;t have a good understanding of what this characteristic is. All we know, for now, is that we&#x27;ve somehow stumbled upon a near-perfect set of heuristics that work amazingly well on the SAT instances we encounter in practice.<p>There&#x27;s also the intriguing possibility that &quot;natural instances&quot; however one might define them arise through some complicated process that ends up sampling from easy SAT instances. I think of it as analogous to the Benford&#x27;s law (see: <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Benford%27s_law#Explanations" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Benford%27s_law#Explanations</a>) except that we do not have nearly as good explanations for SAT solvers.<p>It is also highly likely that MOST instances of SAT are easy but that doesn&#x27;t preclude it from being NP hard, nor does it stop us from easily constructing hard instances. (Such things are absolutely necessary for cryptography as an example.)