In the ideal Platonic society, led by thinking people who use force to train others to become like them, there will be wellbeing because spiritual life, free from the slavery of nature and desires, is the only good life. It means freedom, the satisfaction of knowledge, and peace because the desires and passions of people are the main reason for strife. Also other reasons for strife, such as scarcity, will be eliminated by a planning state taking care of population and birth control. The number of citizens will no longer cause scarcity, envy, territorial expansion and other reasons to go to war.
So Plato started from an initially attractive premise, the importance of a thinking life compared to consumerism, but then issued a whole range of proposals to protect and promote this life which invariably lead to dictatorship. In all this, he is perhaps the classic example of the way in which the combined hostility to nature, materialism and the plurality of society causes hatred for democracy.
But even his premise is questionable. Is solitary reflection of the general, free from appearances and the particular, really the road to wisdom? Perhaps it is more correct to say that sense perception, expression, and hence the use of one’s body and the interaction with other bodies is the best way to gain knowledge. Much of science is still very material, and discussion, argumentation, deliberation and the testing of opinions through expression and discussion protected by human rights can radically improve our opinions.
We need interaction and communication with other people in order to think correctly, and even to think at all. Would we think without our parents and teachers, without speaking and listening to anyone, without engaging in the world of appearances? And would we be able to think more or less correctly without public interaction protected by a democracy and human rights, without venturing in the bigger world of appearances and without leaving our own small and private group of people? Thinking needs the public use of reason (see also this post on Kant). Thoughts are not something you develop on your own, not even in some small and closed group. You first need to listen to as many freely expressed thoughts as possible in order to develop your own thoughts, and then you need to test your own thoughts in confrontation with others.
By making your thoughts public and thus submitting them to scrutiny and tests by other people – first and foremost submitting them to those who are not your private or personal friends, because they might be too kind for you or too like-minded – you are forced to say how you came to have these thoughts and to give an account of the reasons why you have these thoughts instead of others. This will force you to reflect on your reasons and arguments, and, if necessary, to look for better ones. Giving a public account of your reasoning, or knowing in advance that you will give this account, makes you very critical of yourself and helps you avoid mistakes. Nobody wants to make a fool of himself.
The world of appearances, so disliked by Plato for its volatility and imperfection, actually improves the quality of thoughts because of the range of sources of information and opinions, because of the a priori self-criticism that it promotes and because of the a posteriori testing and objecting by other and not necessarily like-minded people (a phenomenon well known in the scientific community).
Giving a public account of your reasoning and arguments, taking objections into account, putting yourself in the place of someone else, think like someone else, look at things from another side or perspective, act as if you hold a contrary point of view, all this is possible only when different perspectives and different points of view are freely expressed. Human rights can help to achieve this. Without human rights, many valuable points of view or perspectives will not be made public, and many valuable objections and counter-arguments will not be known to someone defending a certain thought or idea. This can diminish the quality of the thought or idea in question and therefore it is difficult to understand how a theoretical life can benefit from the elimination of the world of appearances.
Knowledge can hence be defined in a way which is completely different from the Platonic, passive, lonely, anti-social, introvert, non-discursive contemplation. More on the problem of knowledge and politics here.