He spends some time in placing in a ridiculous point of view the attempt to convince a man's understanding and to clear up a doubtful proposition in his mind, by blows.

Undoubtedly it is both ridiculous and barbarous, and so is cock-fighting, but one has little more to do with the real object of human punishments than the other.
One frequent (indeed much too frequent) mode of punishment is death.

Mr Godwin will hardly think this intended for conviction, at least it does not appear how the individual or the society could reap much future benefit from an understanding enlightened in this manner.
The principal objects which human punishments have in view are undoubtedly restraint and example; restraint, or removal, of an individual member whose vicious habits are likely to be prejudicial to the society'; and example, which by expressing the sense of the community with regard to a particular crime, and by associating more nearly and visibly crime and punishment, holds out a moral motive to dissuade others from the commission of it.
Restraint, Mr Godwin thinks, may be permitted as a temporary expedient, though he reprobates solitary imprisonment, which has certainly been the most successful, and, indeed, almost the only attempt towards the moral amelioration of offenders.

He talks of the selfish passions that are fostered by solitude and of the virtues generated in society.

But surely these virtues are not generated in the society of a prison.