In this paper, I inspect the grounds for the mature Spinozist argument for substance monism. The argument is succinctly stated at Ethics Part 1, Proposition 14. The argument appeals to two explicit premises: (1) that there must be a substance with all attributes; (2) that substances cannot share their attributes. In conjunction with a third implicit premise, that a substance cannot not have any attribute whatsoever, Spinoza infers that there can be no more than one substance. I begin the inspection with the analysis of the first premise, which is provided in the form of the four proofs of God’s existence in Ethics Part 1, Proposition 11. While demonstrating how Spinoza adopts a progressive approach, where the fourth proof of God’s existence is more successful and persuasive than the third, which is more successful than the second, etc., I also unpack concepts central to Spinoza’s thinking here, including the concepts of reason (ratio) and power (potesta or potentia). I then analyze the second premise of the Spinozist argument for substance monism, as established by Ethics Part 1, Proposition 4 in conjunction with Ethics Part 1, Proposition 5. I take up and respond to the objection attributed to Leibniz that a substance p can have the attributes x and y and a substance q can have the attributes y and z, and thus that substances can share some attributes while remaining distinct. Throughout the study, my attention is focused on the argumentative procedures Spinoza adopts. This yields a close, internalist reading of the text where Spinoza effectively embraces substance monism. In conclusion to this study, I underscore to the originality of Spinoza’s argument for seventeenth century theories of substance.