Natural philosophy or the philosophy of nature (from Latin philosophia naturalis), is a term applied to the objective study of nature and the physical universe that was dominant before the development of modern science. It is considered to be the precursor of natural sciences such as physics.
Forms of science historically developed out of philosophy or more specifically natural philosophy. At older universities, long-established Chairs of Natural Philosophy are nowadays occupied mainly by physics professors. Modern notions of science and scientists date only to the 19th century (Webster's Ninth New Collegiate Dictionary dates the origin of the word "scientist" to 1834). Before then, the word "science" simply meant knowledge and the label of scientist did not exist. Isaac Newton's 1687 scientific treatise is known as The Mathematical Principles of Natural Philosophy.
In the 18th and 19th centuries, natural philosophy referred to what is now called physical science. From the mid-19th century, when it became increasingly unusual for scientists to contribute to both physics and chemistry, it just meant physics, and is still used in that sense in degree titles at the University of Oxford. Natural philosophy was distinguished from the other pre-cursor of modern science, natural history, in that the former involved reasoning and explanations about nature (and after Galileo, quantitative reasoning), whereas the latter was essentially qualitative and descriptive.
The study of natural philosophy presupposes that change is a reality. Although this may seem obvious, there have been some philosophers who have denied change, such as Plato's teacher Parmenides and later Greek philosopher Sextus Empiricus and perhaps some Eastern philosophers as well. George Santayana in his Scepticism and Animal Faith attempted to show that the reality of change cannot be proven. If his reasoning is sound, it follows that to be a physicist, one must restrain one's skepticism enough to trust one's senses, or else rely on anti-realism.
Beginning with Schelling, the mode of change studied in natural philosophy has been development, rather than evolution. Development is predictable directional change, while evolution is the irreversible accumulation of historically mediated information.
In René Descartes' metaphysical system of dualism, there are two kinds of substance: matter and mind. According to this system, everything which is "matter" is deterministic and natural—and so belongs to natural philosophy—and everything which is "mind" is volitional and non-natural, and falls outside the domain of philosophy of nature.
Boyle, while he is the first to fully embrace such an approach in both his experimental endeavours and his writings, shares with Bacon (and Galileo who was the inspiration in these matters for both Bacon and Boyle) a conviction that practical experimental observation was the key to a more satisfactory understanding of nature than would have otherwise been sought through either exclusive reference to received authority or a purely speculative approach.
Although Galileo's 'natural philosophy' is hardly distinguishable from science in many ways, the connection between his experiments and his writings about them is characteristically philosophical, rather than being cluttered with the results of meticulously recorded observational detail of practical scientific research, in the way that Boyle subsequently advocated.
Even though Boyle described what he practiced as 'natural philosophy', the very innovations that Boyle introduced can be seen as a basis for delineating a transition from proto-science to science. Among these innovations are an insistence upon the publication of detailed experimental results, including the results of unsuccessful experiments; and also a requirement for the replication of experiments as a means of validating observational claims.
Thus Boyle's application of the term 'natural philosophy' to his own work may be regarded an anachronistic conflation with earlier proto-science, since the distinction between the terms 'natural philosophy' and 'science' only arose after Boyle's passing.
Boyle would therefore describe his work as 'natural philosophy', whereas we would describe it as 'science'; and yet Boyle's use was correct for his own time. Nonetheless, he is in many ways the architect of the modern distinction between the two terms.
The ancient emphasis on deduction has its representative in Aristotle's Organum, and the new emphasis on induction and research has its representative in Francis Bacon's treatise Novum Organum.
In Mary Shelley's novel Frankenstein, the protagonist Victor Frankenstein, as a boy, becomes interested in Natural Philosophy, especially in authors like Cornelius Agrippa, Albertus Magnus and Paracelsus. This interest is what lead the young Frankenstein to work on chemistry and biology and finally, to create the wretch.