All science would be superfluous if the outward appearance and the essence of things directly coincided. Das Capitol, Vol III, Ch.48

The ideas of the ruling class are in every epoch the ruling ideas, i.e. the class which is the ruling material force of society, is at the same time its ruling intellectual force. The class which has the means of material production at its disposal, has control at the same time over the means of mental production, so that thereby, generally speaking, the ideas of those who lack the means of mental production are subject to it. The ruling ideas are nothing more than the ideal expression of the dominant material relationships, the dominant material relationships grasped as ideas.

The philosophers have only interpreted the world, in various ways; the point is to change it

Philosophy is to the real world as masturbation is to sex

Philosophy stands in the same relation to the study of the actual world as masturbation to sexual love.

There is no royal road to science, and only those who do not dread the fatiguing climb of its steep paths have a chance of gaining its luminous summits.