Full Idea
The concept of cause always requires that something A be of such a kind that something else B follows from it necessarily and in accordance with an absolutely universal rule. Appearances may give a rule that something usually happens, but not necessarily.
Gist of Idea
Appearances give rules of what usually happens, but cause involves necessity
Source
Immanuel Kant (Critique of Pure Reason [1781], B124/A91)
Book Reference
Kant,Immanuel: 'Critique of Pure Reason', ed/tr. Guyer,P /Wood,A W [CUO 1998], p.223
A Reaction
I must side with Hume when it is put like this. As all empiricists are keen to tell us, a strong feeling of necessity is not enough to guarantee it. Has Kant confused 'natural' and 'metaphysical' necessity? We can't learn natural necessity a priori.