Hume questioned the process of inductive thinking, which had been the hallmark of science. Hume was of the view that no matter how many individual observations an investigator may come up with (empiricism), he would never be in a position to make an unrestricted general statement. "When on innumerable occasions we observe certain experiences succeeding others, we naturally feel under similar circumstances in the future like events or causes will be followed by like effects. ... only custom or habit may validly be said to serve as the foundation for this causal idea."17 There is no guarantee, no matter how accustomed we may have become to certain sequential events of the past that the sequence will necessarily repeat itself. He concluded that the "whole of our science assumes the regularity of nature - assumes the future will be like the past ..."18 This is referred to in the texts as Hume's problem, the problem of induction.