For example, if you decide to build a bookcase, you are unlikely to nail or screw boards together at random in the hope that the finished product will serve your purposes. Instead, you will develop mental representations to guide your efforts, such as a visual image of the finished product and general principles for its successful construction (e.g., “build from the bottom up”).
1. Deductive Reasoning:
Two types of reasoning underlie many of our attempts to make decisions and solve problems. In deductive reasoning, we reason from the top down, that is, from general principles to a conclusion about a specific case.
When people reason deductively, they begin with a set of premises (propositions assumed to be true) and determine what the premises imply about a specific situation. Deductive reasoning is the basis of formal mathematics and logic. Logicians regard it as the strongest and most valid form of reasoning because the conclusion cannot be false if the premises (factual statements) are true. More formally, the underlying deductive principle may be stated: Given the general proposition “if X then Y,” if X occurs, then you can infer Y. Thus, to use a classic deductive argument, or syllogism, If all humans are mortal (first premise), and If Socrates is a human (second premise), Then Socrates must be mortal (conclusion).
2. Inductive Reasoning:
In inductive reasoning, we reason from the bottom up, starting with specific facts and trying to develop a general principle. Scientists use induction when they discover general principles, or laws, as a result of observing a number of specific instances of a phenomenon.
After Ivan Pavlov observed repeatedly that the dogs in his laboratory began to salivate when approached by the experimenter who fed them, he began to think in terms of a general principle that eventually became the foundation of classical conditioning (repeated CS-UCS pairings produce a CR). A college student who experiences repeated negative consequences when she gets drunk may eventually conclude that binge drinking is a high-risk behavior to be avoided. An important difference between deductive and inductive reasoning lies in the certainty of the results. Deductive conclusions are certain to be true if the premises are true, but inductive reasoning leads to likelihood rather than certainty. Even if we reason inductively in a flawless manner, the possibility of error always remains because some new observation may disprove our conclusion. Thus, you may observe that every person named Jordan you have ever met has blue eyes, but it would obviously be inaccurate to reason that, therefore, all people named Jordan have blue eyes. In daily life and in science, inductive and deductive reasoning may be used at different points in problem solving and decision making.
For example, psychologists often make informal observations (e.g., hearing about crime victims like Kitty Genovese who do not receive help when many bystanders are present). These specific observations may prompt them to construct an initial explanation (e.g., diffusion of responsibility) for the observed phenomenon. This is inductive reasoning, so the explanation could be wrong even if it is consistent with all the known facts.
Therefore, scientists move to a deductive process in which they design experiments to formally test specific if-then hypotheses, moving now from a general explanatory principle to a specific observation (the experiment’s results). If the results of these experimental tests do not support their hypotheses, they conclude that their explanation or theory cannot the correct and needs to be revised or discarded.