Scientists operate under the assumption that the laws of nature we discover here and now operate throughout the Universe and have been in effect for all time. James Trefil labels this working assumption, "The Principle of Universality". Although this mode of thought seems obvious today, for much of the history of science, this principle was unknown and would have been considered heretical.
In the "west", our scientific tradition began with the Greek philosophers. The Greeks began a systematic study of reason and often turned their powers of thought to nature. The Greeks operated under the assumption that the rules of nature were different for objects on Earth, and objects in the Heavens. For almost 2,000 years after time of Aristotle (384-322 BC), the study of nature was dominated by the idea that natural processes in the heavens were fundamentally different from those on Earth.
* These notes are based on a wonderful and short book by James Trefil - Reading the Mind of God, In Search of the Principal of Universality, published by Doubleday (Anchor Books) in 1990 (ISBN 0-385-41566-4, LOC: Q175.T72 1990). For many more details, read the book!