Unit One. The Study of Life


1. The Science of Biology


1.5. How Scientists Think


Deductive Reasoning

Science is a process of investigation, using observation, experimentation, and reasoning. Not all investigations are scientific. For example, when you want to know how to get to Chicago from St. Louis, you do not conduct a scientific investigation— instead, you look at a map to determine a route. In other investigations, you make individual decisions by applying a “guide” of accepted general principles. This is called deductive reasoning. Deductive reasoning, using general principles to explain specific observations, is the reasoning of mathematics, philosophy, politics, and ethics; deductive reasoning is also the way a computer works. All of us rely on deductive reasoning to make everyday decisions—like whether you need to slow down while driving along a city street, as in figure 1.5. We use general principles as the basis for examining and evaluating these decisions.


Inductive Reasoning

Where do general principles come from? Religious and ethical principles often have a religious foundation; political principles reflect social systems. Some general principles, however, are not derived from religion or politics but from observation of the physical world around us. If you drop an apple, it will fall whether or not you wish it to and despite any laws you may pass forbidding it to do so. Science is devoted to discovering the general principles that govern the operation of the physical world.

How do scientists discover such general principles? Scientists are, above all, observers: They look at the world to understand how it works. It is from observations that scientists determine the principles that govern our physical world.

This way of discovering general principles by careful examination of specific cases is called inductive reasoning. Inductive reasoning first became popular about 400 years ago, when Isaac Newton, Francis Bacon, and others began to conduct experiments and from the results infer general principles about how the world operates. The experiments were sometimes quite simple. Newton’s consisted simply of releasing an apple from his hand and watching it fall to the ground. This simple observation is the stuff of science. From a host of particular observations, each no more complicated than the falling of an apple, Newton inferred a general principle—that all objects fall toward the center of the earth. This principle was a possible explanation, or hypothesis, about how the world works. You also make observations and formulate general principles based on your observations, like forming a general principle about the timing of traffic lights in figure 1.5. Like Newton, scientists work by forming and testing hypotheses, and observations are the materials on which they build them.




Figure 1.5. Deductive and inductive reasoning.

A driver who assumes that the traffic signals are timed can use deductive reasoning to expect that the traffic lights will change predictably at intersections. In contrast, a driver who is not aware of the general control and programming of traffic signals can use inductive reasoning to determine that the traffic lights are timed as the driver encounters similar timing of signals at several intersections.


Key Learning Outcome 1.5. Science uses inductive reasoning to infer general principles from detailed observation.