  Leibniz has several straightforwardly metaphysical reasons for denying that the mind could be a tabula rasa. First, and most obvious, since there can be no genuine causal interaction among substances, then there could be no way that all our ideas could come from experience; indeed, no ideas could, strictly speaking, come from experience. (Leibniz will, however, adopt a more liberal understanding of sense experience, so that this is not mooted tout court.) But, second, and rarely remarked upon, Leibniz believes that the view that our minds are blank slates at birth violates the Principle of the Identity of Indiscernibles. In short, PII works against qualitatively identical physical atoms and against qualitatively identical (because blank) souls. Further, in one telling passage, he shows us the metaphysical underpinnings of the empiricist view that he finds so objectionable. He writes, “Experience is necessary, I admit, if the soul is to be given such and such thoughts, and if it is to take heed of the ideas that are within us. But how could experience and the senses provide the ideas? Does the soul have windows? Is it similar to writing-tablets, or like wax? Clearly, those who take this view of the soul are treating it as fundamentally corporeal” (A VI vi 110/RB 110). Locke famously entertained the possibility of “thinking matter”, and Leibniz found such a thesis abhorrent. Throughout his career, Leibniz expresses no doubt that the human mind or soul is essentially immaterial, and Locke's skepticism about the nature of substance is fundamentally at odds with Leibniz's most deeply held philosophical commitments. But, of course, the consequence of this is that Leibniz seeks to undermine Locke's position with respect to the origin and nature of ideas. That the mind, according to Leibniz, must be essentially immaterial has been shown above in the section on metaphysics. But Leibniz does have a particular argument for the mind's immateriality or against its mechanism that concerns the nature of thought and ideas. This is his famous metaphor of a mill, which comes forth both in the New Essays and the Monadology. According to Leibniz, perceptions cannot be explained in mechanical or materialistic terms. Even if one were to create a machine to which one attributes thought and the presence of perceptions, inspection of the interior of this machine would not show the experience of thoughts or perceptions, only the motions of various parts.