The most fundamental assumption of science is determinism. . .the belief that everything that happens is determined by other events. A scientist assumes that there is a önatural causeò that explains why this thing happened in this way at this time. Sometimes, the explanation is obscure, and sometimes an element of chance has to be included in understanding events. For example, your very existence was caused by the fertilization of an egg in your mother's body by a sperm from your father's body. However, that particular egg and sperm were only one of many millions that your parents could produce, and hence the fact that you are the particular person you are was determined by chance.
It is this element of chance that distinguishes determinism from pre-determinism. This latter is the belief that everything that ever has or ever will happen is somehow programmed in advance. According to such a view, we are all simply acting out our pre-determined roles while our destinies unfold from day to day. Science does not demand such a fatalistic view of life.
What science does demand is the belief that nature is lawful and that, as scientists discover the laws of nature, we will be able to predict the consequences of various events. Scientific laws are of the form: If event A, then event B. If the law is true, then you can predict the occurrence of B whenever A happens, and you can control the occurrence of B if you can control the occurrence of A.
When a scientific law appears to have very wide generality, it is called a principle. For example, one well established principle of behavior is this: If reward is scheduled at a regular time, organisms tend to postpone responding until shortly before the next scheduled reward. This principle was first clearly demonstrated in research on hungry rats responding for food reward. But it applies to all species of organisms with all kinds of responses for all kinds of rewards. A relevant instance is the behavior of most students. Exams (rewards) are usually scheduled at regular times; hence studying (responding) is typically put off until shortly before the exam.
The behavior sciences attribute behavior to some combination of three general factors: genetic nature, past experience, and present circumstances. The goal of such sciences is to discover what factors are responsible for some behavior of interest. Consider, for example, that I am now interested in your survival as a college student. What are the factors that determine the likelihood of your success? There is no denying that native intelligence is relevant; obviously, educa tional preparation is critical; and various features of the college environment (e.g., size of classes, living arrangements) importantly affect performance. Scientists seek to discover those factors.