"That is, the more expressive a language or system is, the less we can reason about it, and vice versa. The more capable the system, the less comprehensible it is."

What makes these assertions true? Research/data/polls etc would be helpful. It is hard to accept such wide-ranging claims without some proof.

Also, could someone please post the effective definitions of "expressiveness" and "capability" as used in the post ?

Thanks.