Charles Lyell, Historian of Time's Cycle - Time's Arrow, Time's Cycle: Myth and Metaphor in the Discovery of Geological Time - page 120

as James Hutton wrote with admirable directness: "If the stone, for example,
which fell today were to rise again tomorrow, there would be an end of
natural philosophy, our principles would fail, and we would no longer
investigate the rules of nature from our observations" (1795, I, 297).
2. The uniformity of process. If a past phenomenon can be rendered as the
result of a process now acting, do not invent an extinct or unknown cause as
its explanation. This principle bears the confusing name "actualism" in
reference to the meaning of its cognate ( actualisme, Aktualismus ) in most
continental languages—where actual means "present," not "real" as in
English. Hence, actualism is the notion that we should try to explain the past
by causes now in operation. Philosopher Nelson Goodman (1967) recognized
that actualism is little more than geology's own way of expressing a general
rule of scientific methodology, the so-called principle of simplicity: don't
invent extra, fancy, or unknown causes, however plausible in logic, if
available processes suffice.
These two meanings of uniformity are statements about methodology, not
testable claims about the earth. You can't go to an outcrop and observe either
the constancy of nature's laws or the vanity of unknown processes. It works
the other way round: in order to proceed as a scientist, you assume that
nature's laws are invariant and you decide to exhaust the range of familiar
causes before inventing any unknown mechanisms. Then you go to the
outcrop. The first two uniformities are geology's versions of fun- damental
principles—induction and simplicity—embraced by all practicing scientists
both today and in Lyell's time.
But Lyell's other uniformities are radically different in status. They are
testable theories about the earth—proposals that may be judged true or false
on empirical grounds.
3. Uniformity of rate, or gradualism. The pace of change is usually slow,
steady, and gradual. Phenomena of large scale, from mountain ranges to
Grand Canyons, are built by the accumulation, step by countless step, of
insensible changes added up through vast times to great effect (Figure 4.4).
Major events do, of course, occur-