Aktualitetsprincippet

Aktualitetsprincippet (på engelsk: uniformitarianism) er antagelsen om, at de naturlove og processer I naturen som er virksomme i dag, har været gældende og virksomme på samme måde til alle tider og steder.^[1][2] Princippet har at gøre med det metafysiske grundlag for videnskabelig tænkning, fx forestillingen om at forholdet mellem årsag og virkning er uforanderligt i tid og rum,^[3] og at fysikkens love er de samme overalt i universet og til alle tider.^[4] Selv om det er en påstand som ikke kan bevises videnskabeligt, har aktualitetsprincippet været et fundamentalt aksiom i så at sige alle naturvidenskaber.^[5]

Princippet fik i løbet af 1800-tallet stor betydning inden for geologi. I dag anses Jordens historie for at være kendetegnet ved små og hyppige, men langsomme og gradvise forandringer, vekslende med sjældne, kortvarige katastrofer med store konsekvenser.

1. ^ Gordon, 2013: 79
2. ^ Gould 1965, s. 223–228, "The assumption of spatial and temporal invariance of natural laws is by no means unique to geology since it amounts to a warrant for inductive inference which, as Bacon showed nearly four hundred years ago, is the basic mode of reasoning in empirical science. Without assuming this spatial and temporal invariance, we have no basis for extrapolating from the known to the unknown and, therefore, no way of reaching general conclusions from a finite number of observations."
3. ^ Gordon, 2013: 82; "The uniformitarian principle assumes that the behavior of nature is regular and indicative of an objective causal structure in which presently operative causes may be projected into the past to explain the historical development of the physical world and projected into the future for the purposes of prediction and control. In short, it involves the process of inferring past causes from presently observable effects under the assumption that the fundamental causal regularities of the world have not changed over time."
4. ^ Strahler, A.N. 1987. Science and Earth History- The Evolution/Creation Controversy, Prometheus Books, Amherst, New York, USA. p. 194: “Under the updated statement of a useful principle of uniformitarianism it boils down essentially to affirmation of the validity of universal scientific laws through time and space, coupled with a rejection of supernatural causes.” p. 62: “In cosmology, the study of the structure and evolution of the universe, it is assumed that the laws of physics are similar throughout the entire universe.”
5. ^ Simpson 1963, s. 24–48, "Uniformity is an unprovable postulate justified, or indeed required, on two grounds. First, nothing in our incomplete but extensive knowledge of history disagrees with it. Second, only with this postulate is a rational interpretation of history possible, and we are justified in seeking—as scientists we must seek—such a rational interpretation."