1. An inquiry as to what things we know with certainty;
2. An investigation of propositions which it is wise to accept in practice, although they have only probability and not certainty in their favour.

'The grounds of probability,' he says, 'are two: conformity with our own experience, or the testimony of others' experience.' The King of Siam, he remarks, ceased to believe what Europeans told him when they mentioned ice. We therefore believe what conforms to our existing beliefs and expectations based on our experience. This experience is modified by what others tell us. As more people tell us of different ideas, we become more inclined to believe them.

Degrees of Assent
Locke says that the degree assent we give to any proposition should depend upon the grounds of probability in its favour. after pointing out that we must often act upon probabilities that fall short of certainty. We act on what is more likely to be successful, not what will absolutely be successful, because we usually don't know. Locke says that the right use of this consideration 'is mutual charity and forbearance. Since it is unavoidable to the greatest part men, if not all, to have several opinions, without certain and indubitable proofs of their truth; and it carries too great an imputation of ignorance, lightness, or folly, for men to quit and renounce their former tenets presently upon the offer of an argument which they cannot immediately answer and show the insufficiency of; it could, methinks, become all men to maintain peace and the common offices of humanity and friendship in the diversity of opinions, since we cannot reasonably expect that any one should readily and obsequiously quit his own opinion, and embrace ours with a blind resignation to an authority which the understanding of man acknowledges not. For, however it may often mistake, it can own no other guide but reason, nor blindly submit to the will and dictates of another. If he you would bring over to your sentiments be one that examines before he assents, you must give him leave at his leisure to go over the account again, and, recalling what is out of his mind, examine the particulars, to see on which side the advantage lies; and if he will not think over arguments of weight enough to engage him anew in so much pains, it is but what we do often ourselves in the like cases and we should take it amiss if others should prescribe to us what points we should study : and if he be one who wishes to take his opinions upon trust, how can we imagine that he should renounce those tenets which time and custom have so settled in his mind that he thinks them self-evident, and of an unquestionable certainty; or which he takes to be impressions he has received from God himself, or from men sent by him? How can we expect, I say, that opinions thus settled should be given up to the arguments or authority of a stranger or adversary? especially if there be any suspicion of interest or design, as there never fails to be where men find themselves ill-treated. We should do well to commiserate our mutual ignorance, and endeavour to remove it in all the gentle and fair ways of information, and not instantly treat others ill as obstinate and perverse because they will not renounce their own and receive our opinions, or at least those we would force upon them, when it is more than probable that we are no less obstinate in not embracing some of theirs. For where is the man that has incontestable evidence of the truth of all that he holds, or of the falsehood of all he condemns; or can say, that he has examined to the bottom all his own or other men's opinions? The necessity of believing without knowledge, nay often upon very slight grounds, in this fleeting state of action and blindness we are in, should make us more busy and careful to inform ourselves than to restrain others. . . . There is reason to think, that if men were better instructed themselves, they would be less imposing on others.'

Although the above is written in old English, it is well worth study and understanding. It contrast so much with the bigotry, self-seeking, and desire to be right rather than to accept the truth we have seen above and see today. The examples illustrate how irrational beliefs are harmful not only to ourselves but also to those who are under our influence.