The aim of the following investigation is to clarify John Buridan’s conception of quidditative definitions and causal definitions by analyzing the different meanings assigned to these expressions in primarily two of his works on Aristotle: his Commentary on the Topics, an early and little studied work, and his Commentary on the Posterior Analytics. The investigation will show an expansion in the meaning of causal definition in the context of the latter commentary. The search for an explanation for this expansion will lead to an investigation of the role definitions play in the construction of robust scientific demonstrations. In this way, it will be shown that this broadening of what Buridan meant by casual definitions is due primarily to an emphasis on science as something that inquiries into the causal capacities of the things that make up the world.