La Mettrie, a physician, allows only the use of facts and observations derived through experimentation to describe the human body. From this approach he concludes that it is a machine, though admittedly a very complex machine that is initially impossible to get a clear idea of, and therefore to define. It is only the arrangement of our parts that distinguishes one person from another, or a person from an animal, plant, or anything else. Observation leads La Mettrie to conclude that animals think, are intelligent, and feel repentance and shame just as humans. A materialistic explanation can even be extended to illuminate the causes of criminal acts. Indeed, all things have materialistic explanations for La Mettrie. Our abilities as well as our limitations can be ascribed to our material organization; it is our ignorance of them that has led us to use alternative explanations (usually supernatural). For La Mettrie, we exist simply to exist, but if there is a reason, then nature made us for pleasure first and to be happy, and secondarily to be educated (though he suggests that like Gould's classic article about the Spandrels of San Marco, our ability to learn may have only been accidental.)
There is one quote by La Mettrie that I cannot improve on by summarization, so I will include it's translation (full pdf paper):
There is nothing contradictory aboutIn case this translation isn't clear, to La Mattrie, thought is not incompatible with organized matter, rather it is a property of it! Even 262 years later, we haven't solved that riddle yet. But imagine if we really acted as though we were aware that we are the purely physical machines he describes us as. What if we understood that we and all other things differ only in minor differences of organization, without any substantial differences at all. Small changes in organization can lead to large changes in appearance and behavior. Wouldn't we then be more sympathetic to our fellow men and creatures? La Mattrie certainly thought so:
(1) being a machine and (2) being able to feel, to think and to tell right from wrong like telling blue from yellow; that is,
(1) being a mere animal and (2) being born with intelligence and a sure instinct for morality,
any more than there is about
•being an ape or a parrot and •being able to give oneself pleasure.
. . . .Who would ever have guessed in advance that a drop of liquid ejaculated in mating would give rise to such divine pleasure? or that there would be born from it a little creature who would be able one day, given certain laws, to enjoy the same delights? Thought incompatible with organised matter? That is so far from right, I believe, that thought seems to be a property of matter, like electricity, power to move, impenetrability, extension, etc.
Those haughty, vain, self-praising beings who are marked off by their pride more than by the label ‘men’ —are basically only animals and upright-crawling machines....
If only men would always show [as animals] the same gratitude for kindness and the same respect for humanity! Then we wouldn’t have to fear being met with ingratitude, or to fear these wars that are the scourge of the human race and the real hangmen of the law of nature.