Welcome back. In the previous modules, we've worked on building a foundation. First, understanding the range of research activities that are conducted, where research is conducted, how it is supported, who conducts research, and why is it important? Then, in the second module, we set out to understand what it really means for an investigation to be scientific and how to identify when an investigation may not be scientific. This culminated in us outlining the scientific method. In this module, we're going to bring those pieces together. How do we build research programs that are scientific? In order to do that, we'll begin by understanding some basic terminology that's often misused and can cause some confusion. In this lesson, we're going to lay down a lexicon that will establish a formal common language that we can use to describe the scientific research process. In particular, we're going to define four important terms; theory, hypothesis, law, and model. Although our scientific research lexicon is much broader than these four terms, we're going to focus on these four because they're often confused for one another, conflated with one another, and sometimes even used interchangeably. Here we want to establish the formal definitions for each of these terms in order to avoid confusion as we move forward. Let's begin by talking about theories. We start with this term because it's probably the most widely misused term that we will encounter. It's important that we set the record straight right off the bat. According to the US National Academy of Sciences, a theory is a plausible or scientifically acceptable, well-substantiated explanation of some aspect of the natural world. It's an organized system of accepted knowledge that applies in a variety of circumstances to explain a specific set of phenomena and predict the characteristics of as yet unobserved phenomena. Wow, what a mouthful. Let's take some time to digest all of that. Take a few seconds to reread that for yourself, then I'm going to highlight a few important points. Now, let's dig back to our discussion of what it meant for an explanation to be scientific. Recall that a scientific explanation has to be testable, rational, and based on observations. When we say that a theory is a plausible or scientifically acceptable, well-substantiated explanation, this is exactly what we're saying. We're saying that it's based on observations that have been tested and confirmed. It follows rationally from those observations and that it meets these rigorous standards of the scientific method. Also connecting back to our lesson on scientific explanations, we notice that a theory applies in a variety of circumstances. In other words, a theory is general. But we might also notice that a theory is used to explain a specific set of phenomenon. How do we reconcile a theory being both general and specific? Well, it helps to compartmentalize these two things. The explanation is general, but the thing that is being explained is not. Take the theory of gravitation, for example. The thing being explained, gravity, is specific, but the theory of gravitation offers that gravity works the same way throughout the universe. It's not simply that gravity works when you're standing on the surface of the Earth. No, gravity works whether you're in space, on the surface of Mars, or considering the pull between the Earth and the Sun. Next, we say that theories can be used to predict the characteristics of as yet unobserved phenomena. Theories help us not just understand or explain a phenomenon, but explain it well enough that we can predict what will happen if we were to observe that phenomenon under new circumstances. Let's return again to the theory of gravitation. No person has been to Venus or Mercury or some other planet outside of our solar system, but our theory of gravitation is sufficiently well-substantiated that we wouldn't have any reservations in predicting what the pull of gravity would be on one of those planets if we simply know the mass of the planet. Lastly, I want to emphasize that a theory is an organized system of accepted knowledge. This is perhaps where much of the trouble comes in when we think about theories. Theories have to be accepted. As we so often do in our lives, we'll say things like, I have a theory about that, or my theory is, when we do that, we're not describing a theory at all. Even if it's a scientific explanation, it's not a theory because it doesn't become a theory until it has been widely accepted as true. When we use language like that, what we're actually describing is a hypothesis. On that note, let's formally define a hypothesis. According to the US National Academy of Sciences, a hypothesis is a tentative explanation for an observation, phenomenon, or scientific problem that can be tested by further observation. Scientific hypotheses must be posed in a form that allows them to be rejected. Once again, read through this definition on your own and think about it in the context of scientific explanations that we learned about in the previous lesson. Notice first that a hypothesis is a tentative explanation. When we offer a hypothesis, we are offering a possible explanation. But moreover, we're offering a possible explanation of a phenomenon that can be observed and tested. Importantly, we cannot hypothesize about something that we cannot offer a scientific explanation for. For example, we cannot currently hypothesize that there is a God because we cannot observe or test whether there is a God. Finally, we notice that it's so important to recognize that a hypothesis is a tentative explanation that the definition adds the explicit statement that scientific hypotheses must be posed in a form that allows them to be rejected. We're going to spend some time distinguishing theories and hypotheses. But here we want to point out that a hypothesis is offered as a possible explanation, whereas the theory offers an accepted explanation. A theory begins as a hypothesis. It is then tested and observed. If it is accepted, it becomes a theory. That's not to say that theories are not also tentative. It's very possible for a theory to be proven incorrect or incomplete. It is rather distinguished by the fact that the evidence supports the theory to such an extent that it's been widely accepted by the scientific community. We're going to conclude by talking about two related terms, laws and models. Laws and models are useful tools for scientists, but we should be careful to distinguish them from their underlying theories. Let's begin by defining a law. A law is an empirically verified quantitative relationship between two or more variables. Recall that in a scientific inquiry, we're often interested in investigating the relationship between sets of factors or variables. A law offers a mathematical relationship between these variables. Examples include the ideal gas law and Newton's laws of motion, the second law which is shown here. While it's critical to establish laws, and many theories would be incomplete without these laws, a law is not a theory. A law is descriptive. It doesn't offer an explanation for why the variables are related, it simply describes how they are related. A theory, on the other hand, offers an explanation. Finally, a model is a mathematical abstraction of a theory built with reasonable simplifications and assumptions. It's helpful to think of a model as a specific implementation of a theory. Consider, for example, that we wanted to compute the trajectory of a baseball that's launched with a certain velocity in a certain direction. In solving that trajectory, several theories and laws will come into play to varying extents, including the theory of gravitation and conservation of mass, energy, and momentum. When we actually want to compute the trajectory, we need to build a model. That model needs to mathematically establish the conditions under which we'll solve the trajectory and establish any assumptions that come along with that. For example, what external forces will we consider to be acting on the ball, and what mathematical relationship will we give them? Will we consider the drag forces from the air that served to act against the baseball's motion? If so, what mathematical relation do I assume for these forces? There are different options. Once we make these decisions, we've established a model that will allow us to estimate the trajectory of the ball. Models are an important part of scientific exploration, but we should recognize their role as a supporting tool, and moreover, recognize that models are built from idealizations and simplifications. As the famous aphorism from statistician George Box says, "All models are wrong, but some models are useful." The definitions introduced in this lesson were taken from the sources shown here. Additional references can be found on the following slide.