This principle, formulated in 1783 by James Hutton, states that for all events from the past an explanantion may be found in phenomena and forces still in evidence in or upon the earth. It is one of the three principles which forms the basis of the entire geological science, but of stratigraphy in particular, because it not only places a valuable working-method in the hands of the geologist but also lends comprehension and meaning to his observations. At present it is therefore applied in an increasing measure mainly to that part of the geological history which is characterised by the presence of visible forms of life, with this proviso that the rate and the intensity of the geological forces may fluctuate.
Text of inaugural address delivered in the Dept. of Geology, University of Pretoria, on 8 September 1966. Text in Afrikaans, summary in English.