The purpose of this article is to clarify the distinction between primary and secondary qualities in the philosophy of Locke, in the light of the traditions that on the subject had on hand. First, I will contrast his claims about primary and secondary qualities clearly mechanistic approach to the philosophy of Boyle and Descartes's characterization of the extent as a substance of bodies and the thought as a substance of soul. Finally, I will put Locke's position in the light of the experimental philosophy of Newton and his parti cular understanding of the qualities of matter, as is described in his Principia. From this I can conclude that the distinction between primary and secondary qualities in Locke's Essay describes the terms of our understanding of nature, but not of what are the objects themselves. This implies that the distinction is purely operational, rather than a description of the nature of the objects oour knowledge.