In the definite position taken up in Protagoras, it is the faculty of Measuring pleasures against one another and against pains.

In other dialogues, measure is still the important aspect of the process, although the things to be measured are not given.
2.

As regards the Will.

The theory that vice, if not the result of ignorance, is a form of madness, an uncontrollable fury, a mental distemper, gives a peculiar rendering of the nature of man's Will.

It is a kind of Necessity, not exactly corresponding, however, with the modern doctrine of that name.
3.