– Contradiction
– Syllogism
– Causation
– Probabilities
Even when our purpose is to ascertain some general truth, the results of systematic inquiry may have various degrees of certainty. If Logic were confined to strict demonstration, it would cover a narrow field. The greater part of our conclusions can only be more or less probable. It may, indeed, be maintained, not unreasonably, that no judgments concerning matters of fact can be more than probable. Some say that all scientific results should be considered as giving the average of cases, from which deviations are to be expected. Many matters can only be treated statistically and by the methods of Probability.
1) observation
2) reasoning checked by observation and by logical principles
3) memory—often inaccurate
4) testimony—often untrustworthy, but indispensable, since all we learn from books or from other men is taken on testimony
5) the agreement of all our results.
On the other hand, belief is caused by many influences that are not evidence at all:
– desire, which makes us believe in whatever serves our purpose
– fear and suspicion, which (paradoxically) make us believe in whatever seems dangerous
– habit, which resists whatever disturbs our prejudices
– vanity, which delights to think oneself always right and consistent and disowns fallibility
– imitativeness, suggestibility, fashion, which carry us along with the crowd.
All these, and nobler things, such as love and fidelity, fix our attention upon whatever seems to support our prejudices, and prevent our attending to any facts or arguments that threaten to overthrow them