The paper deals with Thomas Reid’s comments against Joseph Priestley’s materialism. Priestley’s mechanical materialism is based on the philosophy of Isaac Newton, especially Newton’s rules of reasoning. Priestley rejected substantive dualism and argumented in favor of a single material substance on the basis of these rules. The paper first presents the form of Priestley’s materialism and then analyzes the individual remarks of Reid. The emphasis is on the notions of truth and causality, which both philosophers interpret differently. The last part of the article analyzes whether Reid’s remarks are relevant or not.