 We cannot acquire any genuine knowledge a priori, then. Mill holds that knowledge can be obtained only by empirical observation, and by reasoning which takes place on the ground of such observations. This principle stands at the heart of his radical empiricism. Sense perceptions are the “original data, or ultimate premises of our knowledge” (System, VII: 7)—knowledge of greater levels of complexity is arrived at solely by inferences from that original data. And, as we shall see, Mill grants the validity of only one kind of inference. “Inference, consequently all Proof, and all discovery of truths not self-evident, consists of inductions, and the interpretation of inductions” (System, VII: 283).