Science is philosophy, I'd like to argue. The original term for science, "natural philosophy", was much more apt to describe what it is. But what is "natural philosophy"?
Natural philosophy never really "started", for the natural desire for humans to inquire and understand the world is intrinsic to our nature, and so in a sense we have always been natural philosophers. Aristotle, however, contributed greatly to our understanding of nature through his observations and philosophy. To Aristotle, natural philosophy was concerned with analyzing the causes behind natural phenomena, such as: why does the sun rise each day? Why does a plant grow from a seed?
In the Medieval world, Aristotle's work was rediscovered in Europe from where it was preserved by the Islamic world, and it was incorporated into Medieval theology. Natural philosophy and theology became the vehicles to understand the world, and together, they were used to describe the world in terms of purposes and connections to higher powers. Everyone learned Aristotelian philosophy, and Aristotelian scholasticism dominated natural philosophy, and indeed, became synonymous with it, until late in the seventeenth century.
During the seventeenth century, natural philosophy was transformed by the Scientific Revolution. Advances in mathematics, experimentation, and technology greatly changed the Medieval worldview. The natural philosophy of the 17th century started to resemble what we now call "science", and was largely spurred by Descartes and Francis Bacon. They focused on extracting truth about nature by theoretical (mathematics and logic) and experimental (observations, telescopes, etc.) means in order to describe a mechanical universe without direct divine intervention. However, they certainly had their own theologies incorporated into their philosophies, though these usually were not theological explanations for natural phenomena.
Natural philosophy continued its spurt of development up to the present (and, of course, it's still going on), evolving our picture of the universe by the works of people such as Newton, Copernicus, Galileo, Darwin, Faraday, Einstein, Bohr, etc.
Although the conception of natural philosophers has changed over the course of the Middle Ages, Renaissance, and especially in the seventeenth century, the pursuit of knowledge about the physical world was not separate to philosophy, but was indeed a kind of philosophy. Natural philosophy was not so strictly classified into disciplines like Genetics, Neuroscience, or Mathematics (many of which our nineteenth century predecessors labelled), but all disciplines were seen in a more united sense than they are now. There is certainly overlap now, but the general division seen in schools does not help to portray the underlying unity between the areas of science. And indeed, it does nothing to show the unity of science with philosophy, both of which might as well be living on different planets. But philosophy is a study of the fundamental nature of the world, of knowledge, and of all of reality. Above all, it is a method of inquiry, of using rational thinking to come to a better understanding of one's self and the world. It is a love of wisdom, as its name in Greek implies. Where science fits into this is that science is a kind of philosophy, namely the philosophy of the natural world, as opposed to ethics or logic (though logic is obviously used as a tool in science).
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