Convulsive motions agitate his legs, so that though he wills it ever so much, he cannot by any power of his mind stop their motion, (as in that odd disease called chorea sancti viti), but he is perpetually dancing; he is not at liberty in this action, but under as much necessity of moving, as a stone that falls, or a tennis-ball struck with a racket.

On the other side, a palsy or the stocks hinder his legs from obeying the determination of his mind, if it would thereby transfer his body to another place.

In all these there is want of freedom; though the sitting still, even of a paralytic, whilst he prefers it to a removal, is truly voluntary.

Voluntary, then, is not opposed to necessary but to involuntary.

For a man may prefer what he can do, to what he cannot do; the state he is in, to its absence or change; though necessity has made it in itself unalterable.
12.