"Constant conjunction" is a phrase used in philosophy as a variant or near synonym for causality and induction. It can be construed to contradict a more common phrase: Correlation is not causation. It is often associated with or in constant concomitance by the philosopher David Hume who used the phrase with great regularity in his discussion of the limits of empiricism to provide an explanation for our ideas of causation and inference. In An Enquiry concerning Human Understanding and A Treatise of Human Nature Hume proposed that the origin of our knowledge of necessary connections arises out of observation of the constant conjunction of certain impressions across many instances. A more modern conception would argue that scientific law is distinguishable from a principle that arises merely accidentally because of the constant conjunction of one thing and another, but there is considerable controversy over what this distinguishing feature might be.