For, having been themselves either long resident in Africa, or very frequently there, their knowledge of it could not be questioned.

Having been concerned also in the trade, it was not likely that they would criminate themselves more than they could avoid.

Writing too at a time when the abolition was not even thought of, they could not have been biassed with any view to that event.

And, lastly, having been dead many years, they could not have been influenced, as living evidences may be supposed to have been, either to conceal or exaggerate, as their own interest might lead them, either by being concerned in the continuance of the trade, or by supporting the opinions of those of their patrons in power, who were on the different sides of this question.
Furnished then in this manner, I began my work.

But no person can tell the severe trial which the writing of it proved to me.