In the conclusion, I assess the contribution that Aristotle’s criticisms of earlier Greek psychology make towards establishing a science of soul, in the sense of justifying, and helping us to understand better, the Hylomorphic Thesis, the Efficient-Final Causal Thesis, the Non-Uniformity Thesis, the Part-Hood Thesis, and the Separability Thesis. These results, I argue, show that Aristotle’s hylomorphic psychology is best understood as a dualistic one, which affirms that plants and non-human animals are constituted by interdependent substances and that human beings are constituted by one dependent substance, the body, and one independent one, the rational soul.