What is the basis for holding that Uniformitarianism is valid?

Hi Iano,I think there some things that still need clarification, and it might come down to this: First, uniformitarianism is a geological concept, and as Bill Birkeland pointed out, modern geologists only hold to 1 and perhaps 2. The other sciences never adhered to the principle of uniformitarianism. Perhaps Lyell's original view of it could have found application in other sciences, but the fact of the matter is that it never did.So you can stop pondering whether the other "historical sciences" still hold to any of Lyell's principles. The fact of the matter is that they never did. This is not to say that such fields as cosmology and archeology did not at one time accept some of the same principles expressed by Lyell, only that they never grouped them under a principle called uniformitarianism.Today, most geologists, most cosmologists, most archeologists, indeed most scientists everywhere, accept principle 1, uniformity of law. Principle 2, uniformity of process, would seem to have variable application depending upon the field of science. Principles 3 (uniformity of rate) and 4 (uniformity of state) are accepted by very few scientists as general principles, though of course these terms are perfectly applicable in certain specific situations.--Percy

The age of the earth is not deduced by assuming uniformitarian principles but by measurement. You're correct that uniformity of rate isn't held in geology, but it is only as a general principle that it isn't held. The reason it isn't held as a general principle is because rates cannot be assumed to be constant.Radioactive decay rates are not assumed to be constant as a general principle. Rather, they have been determined to be constant by experiment, observation and theory. We even know that radioactive decay is due to the nuclear weak force, and that it is therefore based upon fundamental physical laws, which we do assume to be constant throughout all time and space. Uniformity cannot be assumed as a general principle. You can only know something is uniform over time and/or space by measuring it or somehow deducing it from related data. The only principle of uniformitarianism remaining is uniformity of physical laws, and it isn't called uniformitarianism. Extremely few in any field of science would list themselves under the uniformitarian category.--Percy