Humanism, and the debates for and against it, is less a perennial philosophical question, returned to again and again, than a moving target, one that reflects the different political, cultural, and economic situation of the moment. The humanism of the renaissance is not the same humanism that was at the center of debates about Stalin and Marx in the sixties. Moreover, I would argue that the question of the human now is profoundly transformed by the Anthropocene, by the awareness that human impact has had an ecological and geological impact on the planet, transforming it for the worst. This does not mean that old debates and discussions of different humanisms in the history of philosophy are relegated to the dustbin of history--just that they take on a different sense and meaning today. Spinoza and Marx's debates with the humanism of their time take on a different sense today.
One of Spinoza's central critical statements is against the tendency, shared by rationalists and romantics, philosophers and theologians, to view ourselves as a "kingdom within a kingdom." As Spinoza writes,
"Indeed they seem to conceive man in Nature as a dominion within a dominion. For they believe that man disturbs, rather than follows, the order of Nature, that he has absolute power over his actions, and that he is determined only by himself. And they attribute the cause of human impotence and inconstancy, not to the common power of Nature, but I know not what vice of human nature, which they therefore bewail, or laugh at or disdain, or (as usually happens) curse. And he who knows how to censure more eloquently and cunningly the weakness of the human mind is held to be godly."
First, a few words about this passage, filled with the rhetorical fire of the scholia, Spinoza weaves together two forms of humanism, two ways of being a kingdom within a kingdom. The first is that of superiority, of humanity as something more than another thing in the world; the second is that of something less, of something fallen from its place in the natural world. These two ideas, humanity as more than nature and humanity as less, rational virtue and depravity, are two sides of the same coin. What would seem to be opposed in the various oppositions of rationalism to romanticism are closer than they would appear. This image of humanity is as much a philosophical position as it is a matter of everyday common sense. It is a spontaneous ideology. It stems from our basic tendency to be conscious of our desires and ignorant of the causes of things. These two things, desires and causes, become two different things, different kingdoms, one governed by causes and the other by our supposed free will.
It is primarily as an ideology, an inadequate idea that Spinoza critiques this humanism. It is a way of thinking that makes it difficult to grasp not only what is true, that we are part of nature, but most beneficial. It is only by understanding ourselves as part of nature, as determined like all other things, that we can actively change and improve our condition, rather than alternatively celebrate and bemoan it, by seeing ourselves as part of nature we can transform our nature.