Julien Offroy de la Mettrie’s “Man a Machine” is a philosophical discussion of the nature of man, free will, and life itself. Mettrie expands on the ideas of several other philosophers before him, but his main premise is the idea that humans are, as the title suggests, essentially machines. He argues against the idea of a soul, and states that, since human behavior can be accurately predicted since we must nourish ourselves and for several other reasons, we are essentially robots. The ideas that Mettrie expresses in this work would eventually gain traction in the early twentieth century with the introduction of quantum mechanics, as scientists explored the idea of duality in much more detail due the the wave-particle duality present in nature.

In this short work, Mettrie begins by arguing that the revelation of the nature of God and the soul of man in the Bible are false—or, at the very least, misinterpreted. Mettrie outlines a list of things that he believes prove the unification of the soul and the physical body. He begins with a discussion of sleep and disease and his idea that, when the physical body is affected in some way, the soul is likewise affected—in disease, people's temperaments change, and they are filled with sloth, and in sleep, the soul is unable to move and express itself, just as the body is.

He then expresses how animals, though they have no soul according to tradition and the beliefs of western religion, still are able to express emotion and personality. His final conclusion is that the soul is actually the result of physical constructs, and therefore, there is no "human soul." He argues that "diverse states of the soul are always correlated with those of the body." However, as science and psychology have proven in recent centuries, the aspects he seems to be discussing are more related to the mind or the personality, which he may subconsciously link with the soul; however, according to modern understanding, they are their own entities entirely.