Hence, ghosts are outside the purview of science, and we cannot study their existence (or lack thereof) with the tools of science. Ghosts, for example, are supernatural entities without a basis in the physical universe and so are not subject to the laws of that universe. This predictability means that we can test hypotheses about natural entities by making observations. Natural things behave in predictable ways - though we may not yet fully understand them - which have observable outcomes. In practice, what’s natural is often identified by testability. Photo credit (left to right) top: Flickr user Todd Lappin, Routing by Rumor, NASA, Dom Hart bottom: MRI photo from NIH, Flickr user Jairo, Wikimedia. Science can study such natural things as bridge engineering, artificial sweeteners, robots, the stock market, the human smile, and magnetic resonance imaging (MRI) technology. So even though we might not think of them as “natural” (in the everyday sense of the word), science can study things like the human smile, human decision-making, artificial sweetener, and learning algorithms for robots because they are part of the physical universe around us. This includes matter, the forces that act on matter, energy, the constituents of the biological world, humans, human society, and the products of that society. Within science, the term natural refers to any element of the physical universe - whether made by humans or not. Ad agencies might try to sell us on “all-natural whole grain goodness,” “natural homeopathic remedies,” or “non-toxic, all-natural household cleansers” - but what does natural really mean? In everyday language, the word natural is often used to describe goods that are wholesome or not made by humans, but in the language of science, natural has a much broader meaning.

What is natural science