“I believe I have discovered a fundamental law to which [human intelligence] is subjected from an invariable necessity, and which seems to me to be solidly established, either by rational proof drawn from a knowledge of our nature, or by the historical test, an attentive examination of the past.  This law is that each of our principal conceptions, each branch of our knowledge, passes successively through three different theoretical states: the theological or fictitious, the metaphysical or abstract, and the scientific or positive.” 

A General View of Positivism (1844): 

     “A Positivism becomes, in the true sense of the word, a religion; the only religion which is real and complete; destined therefore to replace all imperfect and provisional systems resting on the primitive basis of theology.” 

     “Three different sources contribute to produce this elementary moral state--the race, the surroundings, and the epoch.” 

     “I will here give a brief sketch of the progress of opinion on the Origin of Species. Until recently the great majority of naturalists believed that species were immutable productions, and had been separately created. This view has been ably maintained by many authors. Some few naturalists, on the other hand, have believed that species undergo modification, and that the existing forms of life are the descendants by true generation of pre existing forms. Passing over allusions to the subject in the classical writers (Aristotle, Physicae Auscultationes 2.8.2), after remarking that rain does not fall in order to make the corn grow, any more than it falls to spoil the farmer's corn when threshed out of doors, applies the same argument to organisation; and adds (as translated by Mr. Clair Grece, who first pointed out the passage to me), "So what hinders the different parts (of the body) from having this merely accidental relation in nature? as the teeth, for example, grow by necessity, the front ones sharp, adapted for dividing, and the grinders flat, and serviceable for masticating the food; since they were not made for the sake of this, but it was the result of accident. And in like manner as to other parts in which there appears to exist an adaptation to an end. Wheresoever, therefore, all things together (that is all the parts of one whole) happened like as if they were made for the sake of something, these were preserved, having been appropriately constituted by an internal spontaneity; and whatsoever things were not thus constituted, perished and still perish." We here see the principle of natural selection shadowed forth . . . .” 

Theses on Feuerbach (1845):

     “The philosophers have only interpreted the world differently, what matters is to change it.”  

Toward a Critique of Hegel's Philosophy of Right (1843):

     “Religion is the sigh of the oppressed creature, the heart of a heartless world, just as it is the spirit of spiritless conditions.  It is the opium of the people.” 
     “To abolish religion as the illusory happiness of the people is to demand their real happiness.”  

Socialism: Utopian and Scientific (1880):

     “The proletariat seizes the public power, and by means of this transformati
on the socialized means of production, slipping from the hands of the bourgeoisie, into public property . . . .  Man, at last the master of his own form of social organization, becomes at the same time the lord over nature, his own master-free.” 

     “In what way do proletarians differ from slaves? The slave is sold once and for all; the proletarian must sell himself daily and hourly.  The individual slave, property of one master, is assured an existence, however miserable it may be, because of the master's interest.  The individual proletarian, property as it were of the entire bourgeois class which buys his labor only when someone has need of it, has no secure existence.  This existence is assured only to the class as a whole. The slave is outside competition; the proletarian is in it and experiences all its vagaries.  The slave counts as a thing, not as a member of society.  Thus, the slave can have a better existence than the proletarian, while the proletarian belongs to a higher stage of social development and, himself, stands on a higher social level than the slave. The slave frees himself when, of all the relations of private property, he abolishes only the relation of slavery and thereby becomes a proletarian; the proletarian can free himself only by abolishing private property in general.”