What was the most profound vision of the world in the 18th century? One finds it in Kant, without whom it would be impossible to know the development of consciousness through the centuries. Philosophy is needed for a global view of cultures.
It is important for writers.
For Schopenhauer, life is a continuous, culpable malaise.
Schopenhauer formulates an artistic theory, which, for me, is the most important of all. And, just between us, the extremely naïve and incomplete manner of dealing with art in France is due primarily to the ignorance of Schopenhauer.
Nietzsche, like Kant and Schopenhauer, was Polish! 
Kierkegaard was a Danish pastor, a great admirer of Hegel. Suddenly, he declared war on him, in one of his most dramatic moments.
A sad writer who thinks himself master of reality is a ridiculous thing. Hah! Hah! Hah! Phew!
Marx said that philosophy must not be aristocratic, that is, done by men outside of communal life, but must be done on the scale of the average man, of the man who has needs and lives in society.
And now we move on to the ‘sweets,’ that is, to Revolution.
We are subject to other people’s points of view. Naturally it is necessary to recognize the existence of others. It is an obvious fact. Sartre does not find any philosophical reasons to justify it. The view of others takes away our freedom, defines us. For the other, we are a thing, an object, we have a character, etc. This view of others is contrary to our freedom, but it is only in recognizing the other’s freedom that I free myself from his gaze. All of Sartrean morality consists of recognizing and of affirming freedom.
Guide to Philosophy in Six Hours and Fifteen Minutes [Trans. Ivry]