Critical thinking helps to ensure that our actions and beliefs are rational, that they take account of the way things actually are. So someone who acts rationally has good reasons for doing what they do. They have considered the possible consequences of their actions, and have concluded that their actions are likely to achieve their goals. Equally, a rational person bases their beliefs on evidence and ensures that their beliefs are both logical and consistent with each other. Critical thinkers are thus guided by reason in both their actions and in their beliefs.
Rational beliefs
We ought to have objective reasons for our beliefs, and these reasons may include both evidence and argument. For example, I believe that my friend is not in Mombasa because I saw him in Nairobi only a few minutes ago and he cannot possibly have travelled to Mombasa in that time. Although it is difficult to be entirely objective, critical thinking of this kind ought to be compelling. Anyone presented with the same evidence and the same argument should reach the same conclusion.
Rational people base their beliefs on reason and evidence. Dark clouds are evidence that it is likely to rain and this may be a reason to stay indoors. But reasoning always involves making judgements. Although dark clouds often mean that it is going to rain, we may judge that it will not rain on this particular occasion. Even so, we should weigh up all the relevant evidence and arguments before coming to a conclusion. We should also be willing to suspend belief if there is no good reason to believe one thing rather than another.
Deductive reasoning
Some arguments have conclusions which follow logically from their premises. For example, if it is true that ‘All flamingos are pink’ and also true that ‘This bird is a flamingo’ then the conclusion ‘This bird is pink’ follows as a matter of logic. The truth of the premises are preserved in the conclusion. But some arguments in this form can be ‘valid’ but not sound. An example is: ‘Everyone who eats beans is a soldier’ and ‘William eats beans’ therefore ‘William is a soldier’. The argument is unsound because the first premise is false. Even deductive reasoning relies on facts.
Inductive reasoning
Some arguments draw a conclusion from a body of evidence on the assumption that the future will be like the past. For example, we have observed that dark clouds often cause it to rain, so we conclude that the dark clouds now overhead are also likely to cause it to rain. Logically speaking, no inductive argument can actually prove that something is true. The conclusion of an inductive argument is at best probable rather than certain, especially if the sample of observations underpinning the argument is unrepresentative or too small. Inductive arguments provide degrees of probability rather than certainty.
Abductive reasoning
It is often reasonable to accept the ‘best available’ or ‘most likely’ conclusion that can be derived from a set of observations. We have learnt from experience that the grass gets wet when it rains. So if the grass in a particular place is wet, we can abduce that it has rained. But this is merely the most plausible explanation. The grass might be wet because someone has watered it, or it might be wet from dew – or for some other reason which we have not thought of.
According to the principle known as ‘Occam’s razor’, we should prefer the explanation that requires the fewest assumptions. The scientist Isaac Newton expressed this by saying, ‘We are to admit no more causes of natural things than such as are both true and sufficient to explain their appearances.’ In other words, a good explanation is as simple and as elegant as possible while still explaining everything that needs to be explained.