Inductive and Deductive Reasoning

Inductive and Deductive Reasoning

Inductive reasoning- The Empirical Approach  

Inductive reasoning starts from specific observations or sensory experiences and then develops a general conclusion from them. For example, All the giraffes that I have seen (Repeated observations) have very long necks. Therefore I conclude that all giraffes have long necks. Induction was the earliest and, the commonest popular form of scientific activity. We use it every day in our normal lives as we learn from our surroundings and experiences. We come to conclusions from what we have experienced and then generalize from them, that is, set them up as a rule or belief. 

Francis Bacon stated that one should consult nature, and not rely on the writings of ancient philosophers such as Aristotle or on the Bible. The scientific revolution in the seventeenth century was based on this approach, led by such scientists as Galileo and Newton (remember the apple that fell on his head from the tree that led to his theory of gravity?).Mendel’s discovery of genetics and Darwin’s theory of evolution are perhaps the most famous generalizations in the form of theories that are, even by them, claimed to be developed through inductive reasoning. However there are problems with induction. The first is the question of how many observations must be made before we can reasonably draw a conclusion that is reliable enough to generalize from, and the second is how many situations and under which conditions should the observations be made so that true conclusions can be reached? These problems do not stop us from using inductive reasoning every day quite successfully without even thinking about it. But we should be aware that what might at first seem obvious may not be so reliable with making further investigations. Therefore, in order to be able to rely on the conclusions we come to by using inductive reasoning, we should ensure that we make a large number of observations, we repeat them under a large range of circumstances and conditions and that no observations contradict the generalization we have made from the repeated observations. 

Deductive reasoning- The Rationalist Approach

Deductive reasoning begins with general statements (premises) and, through logical argument, comes to a specific conclusion. Again, a simple example will provide a guide to how this works. All living things (General statement – first premise) will eventually die. This animal is a living thing. (Inference – second premise) Therefore, this animal (Conclusion) will eventually die. This is the simplest form of deductive argument, and is called a syllogism. As you can see it consists of a general statement (called the first premise), followed by a more specific statement inferred from this (the second premise), and then a conclusion which follows on logically from the two statements 

Deduction, as with many philosophical ideas, was first discussed as a way of reasoning by the Ancient Greeks, in particular, Plato. Enquiry is guided by the theory which precedes it. Theories are speculative answers to perceived problems, and are tested by observation and experiment. Whilst it is possible to confirm the possible truth of a theory through observations which support it, theory can be falsified and totally rejected by making observations which are inconsistent with its statement. In this way, science is seen to proceed by trial and error: When one theory is rejected, another is proposed and tested, and thus the fittest theory survives. In order for a theory to be tested, it must be expressed as a statement called a Hypothesis. The essential nature of a hypothesis is that it must be falsifiable. This means that it must be logically possible to make true observational statements which conflict with the hypothesis, and thus can falsify it. However, the process of falsification leads to a devastating result of total rejection of a theory, requiring a completely new start. Another problem with deductive reasoning is that the truth of the conclusions depends very much on the truth of the premise on which it is based. For example, in the past many conclusions about the movement of the planets were incorrect due to the premise that the earth was the centre of the universe.


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