Why do octopuses matter to philosophy? They matter to the part of philosophy concerned with the mind. To see why, we step back and think about the evolutionary connections between all living things. Biologists think of these relationships in terms of a tree of life. This is a huge tree-like pattern, marking which species are close relatives and which are distantly connected. The vertebrates form one branch of the tree, and that is where we find nearly all the animals with large and complex brains. These include ourselves, other mammals, and birds. In evolutionary terms, these are all cousins. In the huge area of the tree containing other animals, invertebrates, there is only one small branch where we also find large brains. This branch contains the cephalopods – octopuses, cuttlefish, and squid. Large nervous systems evolved separately on these two branches, and nowhere else. Octopuses are a separate experiment in the evolution of the mind. Meeting an octopus is like meeting an intelligent alien. So what did this experiment produce? Here is one thing. The nervous system of an octopus is less centralized than ours. In fact, more than half of the octopus' neurons are not in the animal's central "brain" at all, but in the eight arms. It is as if each arm has a mind of its own. Or perhaps in an octopus we see intelligence without a unified self.