**What makes good
reasoning? **

Often the thing that comes to mind when we think about reasoning is logic. Logic has many systems generally stemming from the Greek philosopher Aristotle. We can talk of categorical or propositional or predicate logic. Here are some typical examples

of categorical logic:

All A's are B's and all B's are C's - Therefore all A's are C's

Some A are B. No B are C. - Therefore some A are not C.

of propostional logic:

If there is an F on the sheet of paper there is an L. If there is not an L on the paper there is a V. Therefore there is an F or there is a V.

Predicate logic combines these two and as we get deeper into the study of the logical use of language we come to semantics where we are no longer explaining the use of language but the meanings of words become restricted by limiting their use to meanings consistent formal definitions.

This type of reasoning, which I will call 'formal' logic, is only concerned with reaching conclusions with certainty from certain accepted statements. It is not nearly enough to describe the process of reflection because most evidence we get is not absolutely certain. To think logically in such a way that you follow the formal logical constructs mentioned here would also not be enough because it doesn't cover the search process. Good thinking is as much about finding the evidence in the first place as it is about deriving conclusions from it.

All of scientific knowledge is based on reasoning from imperfect knowledge; indeed none of what we know is 100% certain. We have to deal with probabilities and hypotheses. This gets close to a related issue, which is whether we can ever be confident that any statement we make about reality is absolutely true. We cannot. - Human knowledge is inherently imperfect and formal logic doesn't consider this.

Our 'beliefs' are generally formed by hypothesis testing and on the balance of probabilities. This can become subject to more rigorous tests than simple guidelines for good thinking in a search process. Sometimes a judgement on the balance of probabilities is clearly dependent on definite factual observations that frequency determines the probability or that the events are logically exchangeable. (e.g. tossing a coin and getting heads or getting tails). However, often judgement of the weight given to individual items of evidence may be a matter of personal opinion or choice. This latter situation is the case in most everyday situations; we justify our probability judgements based on the weight we give to certain pieces of evidence.

Whether this aspect of thinking is good or bad is dependent on a number of ways that our reasoning can go astray.