Marx's "Theses on Feuerbach" challenges a competing version of materialist philosophy on the basis that it is both flawed and ineffective. In particular, the argument is directed toward Feuerbach's conceptions of human beings and society. According to the critiques in this text, Feuerbach's view of society is one that separates human social reality from material reality, and characterizes both humans and society as a static objects. Society is a thing with a definite nature, rather than a dynamic set of circumstances that both shapes human beings and is actively shaped by them. For Marx, such conceptions both incorrectly perceive the world and human society, and preclude the possibility of action to make a better world than the one we already have.
The text consists of eleven somewhat self-contained but interconnected ideas. This organization seems meant to clearly outline the logic behind the argument, and the clearly outlined structure helps to convey the attempt at an appearance of scientificity behind Marx's philosophical project. The connections interlinking each step of the argument however are not always explicit, and it seems the reader is meant to do the work of recognizing these connections on his/her own.
The primary critique is that "the thing, reality, sensuousness, is conceived only in the form of the object or of contemplation, but not as sensuous human activity, practice, not subjectively." Viewing the world as an "object" rather than an activity restricts philosophical inquiry into our world to mere description of the nature of things, rather than granting human beings a role in its creation. While versions of materialism allow for people to be shaped by the circumstances of the world, they don't allow for the world to be shaped by people. For Marx, people are changed by their circumstances, but people also actively change the circumstances of the world. He suggests the use of "revolutionary practice," the "coincidence of the changing of circumstances and of human activity or self-changing," or, in other words, the intentional engagement of people in the world to change the social circumstances in which we find ourselves.
Another important critique of Feuerbach's brand of materialism is directed toward his view of the essence of humanity, which Marx calls a "dumb generality." The human essence is seen as those characteristics that can be located in all humans, which is "dumb" because it is incapable of really saying anything. For instance, the "religious sentiment" is conceived as one inherent feature of a human being, and such inherently human features constitute the essence of humanity. The obvious falsehood of a natural "religious sentiment" of all humans aside, this way of talking about human nature is akin to saying the essential nature of humans is that we all have skin and hair. What does this way of thinking actually accomplish? It does not enable the kind of critical project that Marx would like to engage in. Instead, conceiving of humans in this way leads to other flawed conceptions, like "civil society," which imagines society as a definite thing that occurs when you put a bunch of definite "single individuals" together. If single individuals and civil society are definite things with definite natures, we ask questions like "What is society?" rather than questions like "What possibilities exist for changing it?" Instead of "civil society," Marx suggests we should conceive of human life as "social humanity" which is something more like a collectively made world than a definite object that can be ideally described.
The real essence of humanity for Marx is "the ensemble of the social relations." The human essence is not a set of isolable characteristics located in each human individual, but the big picture produced by the view of a collectivity of all humans living in a shared world. Since this world is changeable, there is no static human essence. And a feature such as the "religious sentiment" is not an inherent characteristic of humans, but is itself a "social product," produced by this "ensemble of social relations." Feuerbach gets lost chasing the nature of humans as a set of shared characteristics and misses that the characteristics he locates are not natural, but produced.
Ultimately, Marx's critique is directed at the nature of this brand of philosophy as a whole: "The philosophers have only interpreted the world, in various ways; the point is to change it." For Marx, Feuerbach's kind of philosophy is a purely descriptive project, which both fails to actually describe anything, and fails to enable action. Revolutionary practice is an active, prescriptive project, which asks not "What is the essential nature of the world?" but "How can we make a world that we want to have?" The previous kind of philosophical project, in its insistence on accurate description of a definite, fixed nature of reality, excludes the possibility of such prescriptive or world-making projects. And it is through this revolutionary practice that one can come to find truths that actually help us work toward something, rather than studying the society-thing as if we are gazing into a fish tank:
"The question whether objective truth can be attributed to human thinking is not a question of theory but is a practical question. Man must prove the truth -- i.e. the reality and power, the this-sidedness of his thinking in practice. The dispute over the reality or non-reality of thinking that is isolated from practice is a purely scholastic question."
Marx rejects that purely philosophical questions about whether human ideas represent objective reality are useful questions, the truth or falsehood of a human idea can be demonstrated through real-world practice. An idea is true if you can prove it through demonstration. This also shows the scientificity of Marx's thinking, if we take the essence of science to be something like the attempt to find conceptions that work through active, critical experimentation. But perhaps it goes beyond this kind of scientific inquiry, in suggesting also that we can make our ideas true by making the world in the image of our ideas? If we can prove the truth through practice, and the point of this practice is not to merely interpret the world, but to change it, then we can make the world that we desire into the true world.