Ashley asked:

Assuming that all knowledge is not just a matter of opinion (because if it were, all philosophy would cease!), how do you account for the persistence of different religions, moralities and political ideals?

Answer by Jürgen Lawrenz

You make too easy for yourself. What is ‘knowledge’? This is what you should ask yourself. When you bite into an apple, you do so on the ‘opinion’ that it is good, wholesome and healthy for yourself. Sometimes you can be wrong; it may be a plastic apple. But on the whole this is knowledge. Much the same applies to the earth rotating around the sun and uranium being available for enrichment to make atom bombs. In these and millions of other cases, knowledge and opinion are the same. They are knowledge because, in principle, everyone can make the appropriate tests and get the same results an can therefore hold the ‘correct opinion’. So knowledge is also a consensus among virtually all people who exist, who have the same experience or can acquire it. However, opinion without this backing of experiment and consensus behind it is not knowledge.

Your opinion that maybe a particular picture or song is beautiful can be contradicted by any other party, because taste is not a matter for generalisation. Religions, morality and political ideas fall into the same class. What kind of spirits might rule the world is a matter for each person to decide, because there are no tests to establish the factuality of these opinions, and pretty much the only way to achieve consensus is to persuade people who are already inclined to such beliefs or, if they are not, to force them. Same with morality. And as far as politics are concerned, they are grounded in the simple psychology that most people wish to be free and pursue their own lives without being slaves to anyone else, or running the risk of being mugged or murdered and of course, that they wish to own something and not have others take it away. Plainly none of these issues can be solved except along the lines of common consent among a group of people, and this may differ from one group to another.

So much for the general aspect of your question. It should suffice by itself, except that you seem to labour under a misimpression that ‘philosophy would cease’ if knowledge was just opinion. What I say to you is: you must move on from Aristotle. Even Aristotle’s take on ‘knowledge’ is not the same as yours. He knew about the limits of knowledge and was, for example, content to acknowledge that infinite regress cannot be defeated and puts up an insurmountable barrier to knowledge. This does not mean that we must rely on mere opinion. Leibniz, for example took this one step further and deduced from phenomena that even in infinite regress, the unknowns must be ordered the same as the knowns. But this must do for now. I would recommend, however, that you set yourself an issue of knowledge and read how a real philosopher struggles with it in order to preserve some kernel of its truth. For example, a good exercise would be to read Bacon on the organisation of knowledge, follow it up with Hume’s critique of induction, then maybe Popper’s attempt to save induction, and ending up with Wittgenstein’s Tractatus. That would be wholesome tonic against taking certain issues for granted!