The thumb is not a finger?

It's one of those "which one of these is not like the others".

The thumb contrasts with each of the (other) four by being the only finger that:
* Is opposable
* Has two phalanges rather than three
* Has its inmost phalanx so close to the wrist
* Has much greater breadth and stubby proportions
* Is attached to such a mobile metacarpus (which produces most of the opposability)

The thumb, unlike other fingers, is opposable, in that it is the only digit on the human hand which is able to oppose or turn back against the other four fingers, and thus enables the hand to refine its grip to hold objects which it would be unable to do otherwise. The opposable thumb has helped the human species develop more accurate fine motor skills.