David Hume, answered from the texts and cited to the page.
When we observe one event followed by another, we observe only the events themselves and their sequence.1 We do not observe any necessary connection between them, no matter how carefully we attend. The idea of necessary connection cannot therefore be derived from any single observation of cause and effect — yet we have such an idea, and the question is from what impression it is derived.234
The answer: from the impression of habitual transition. After many constant conjunctions of events of one kind followed by events of another, the mind forms a habit of expecting the second when it perceives the first; this expectation is felt as a kind of internal compulsion, and it is this internal felt necessity that is the source of our idea of necessary connection.
Causation, on the level of what we can demonstratively or experientially know, is constant conjunction in the world plus the inferential habit it produces in the mind. The radical conclusion is not that there is no causation but that the necessary-connection element of our idea of causation has its source in custom, not in observation of any objective bond between events.5
All events seem entirely loose and separate. One event follows another; but we never can observe any tie between them. They seem conjoined, but never connected.An Enquiry Concerning Human Understanding, p. 23
We then call the one object, Cause; the other, Effect. We suppose that there is some connexion between them; some power in the one, by which it infallibly produces the other, and operates with the greatest certainty and strongest necessity.An Enquiry Concerning Human Understanding, p. 23
All these phaenomena lead directly to the precedent system; nor will it ever be possible upon any other principles to give a satisfactory and consistent explication of them. Without considering these judgments as the effects of custom on the imagination, we shall lose ourselves in perpetual contradiction and absurdity. > Of the idea of necessary connexion > 1.3.14 Sect. 14.A Treatise of Human Nature, p. 105
The degree and direction of every motion is, by the laws of nature, prescribed with such exactness that a living creature may as soon arise from the shock of two bodies as motion in any other degree or direction than what is actually produced by it. Would we, therefore, form a just and precise idea of necessity, we must consider whence that idea arises when we apply it to the operation of bodies.An Enquiry Concerning Human Understanding, p. 26
A man, who gives orders for his dinner, doubts not of the obedience of his servants. In short, as nothing more nearly interests us than our own actions and those of others, the greatest part of our reasonings is employ'd in judgments concerning them.A Treatise of Human Nature, p. 261
The most famous Hume thesis and the founding move of modern empiricist philosophy of science. Kant credited the awakening from dogmatic slumber to this argument and developed his own response in the Second Analogy of the Critique of Pure Reason. Twentieth-century logical positivism inherited the regularity account; David Lewis's Humean supervenience program is its most rigorous modern descendant. The "New Hume" debate (Galen Strawson, Edward Craig, Helen Beebee) argues that Hume actually believed in causal powers in nature and was skeptical only about our knowledge of them; the "Old Hume" regularity reading remains dominant. The position structures contemporary debates about laws of nature, induction, and causal realism.