The world was to Shakespeare a great stage of fools on which he was utterly bewildered. His pregnant observations of life are not coordinated into any philosophy.
If you demand my authorities for this and that, I must reply that only those who have never hunted up the authorities as I have believe that there is any authority who is not contradicted flatly by some other authority.
Everything I eat has been proved by some doctor or other to be a deadly poison, and everything I don't eat has been proved to be indispensable for life. But I go marching on.
You do not settle whether an argument is justified by merely showing that it is of some use. The distinction is not between useful and useless experiments but between barbarous and civilized behaviour. Vivisection is a social evil because if it advances human knowledge, it does so at the expense of human character.