We may observe, that there is no species of reasoning more common, more useful, and even necessary to human life, than that which is derived from the testimony of men, and the reports of eye-witnesses and spectators. This species of reasoning, perhaps, one may deny to be founded on the relation of cause and effect. I shall not dispute about a word. It will be sufficient to observe, that our assurance in any argument of this kind is derived from no other principle than our observation of the veracity of human testimony, and of the usual conformity of facts to the reports of witnesses. It being a general maxim, that no objects have any discoverable connexion together, and that all the inferences, which we can draw from one to another, are founded merely on our experience of their constant and regular conjunction; it is evident, that we ought not to make an exception to this maxim in favour of human testimony, whose connexion with any event seems, in itself, as little necessary as any other.

(a) Standardize the argument (hint: it consists of one premise and a conclusion)

1.We may observe, that there is no species of reasoning more common, more useful, and even necessary to human life, than that which is derived from the testimony of men, and the reports of eye-witnesses and spectators.

2.. It will be sufficient to observe, that our assurance in any argument of this kind is derived from no other principle than our observation of the veracity of human testimony, and of the usual conformity of facts to the reports of witnesses.

is this right?

How can you reconcile the idea that the testimony of witnesses is infallible with this conclusion: ??

It being a general maxim, that no objects have any discoverable connexion together, and that all the inferences, which we can draw from one to another, are founded merely on our experience of their constant and regular conjunction; it is evident, that we ought not to make an exception to this maxim in favour of human testimony, whose connexion with any event seems, in itself, as little necessary as any other.

Yes, that is correct. The argument can be standardized as follows:

Premise 1: There is no species of reasoning more common, useful, and necessary to human life than the one derived from the testimony of men and the reports of eyewitnesses and spectators.

Premise 2: Our assurance in any argument of this kind is derived from our observation of the veracity of human testimony and the usual conformity of facts to the reports of witnesses.

Conclusion: Therefore, our assurance in arguments based on human testimony is justified.