Many members of the Society, including Reid, Campbell, and Gregory, were great admirers of Francis Bacon, so the group's aim directed toward the exploration of the sciences of the mind.

After completing The Philosophy of Rhetoric (1776), Campbell published several sermons and finished his lifelong ambition, The Four Gospels, Translated from the Greek (1789).

Campbell was clear that there were seven circumstances involved in a person's decision to act on their impulses as described by The Rhetoric of Western Thought.

The first is probability, the second is plausibility, the third is importance, the fourth is proximity of time, the fifth is connection, the sixth is relation, and the last is interest in the consequences.

While Campbell's literary life was dominated by pedagogical and pastoral concerns, it is apparent that his mind was tempered by the values of the Enlightenment.

Throughout Campbell's literary career, he focused on enlightened concerns such as rhetoric, taste, and genius—perhaps a result of his time in the Aberdeen Philosophical Society.

His attempt to align rhetoric within the sphere of psychology resulted from Francis Bacon's survey of the structure and purpose of knowledge.

The Philosophy of Rhetoric illustrates the Baconian influence of inductive methodology but also scientific investigation—two major concerns of the Enlightenment.

This is seen throughout his writing, with particular emphasis on placing methodology before doctrine, critical inquiry before judgment, and his application of tolerance, moderation, and improvement.

Campbell embraced the philosophical empiricism which John Locke established in An Essay Concerning Human Understanding.

Campbell, like most theorists of the Enlightenment, believed in a universal human nature: the "general principles [of taste] are the same in every people".

[3] He gives the example of tropes and figuration which "are so far from being the inventions of art, that, on the contrary, they result from the original and essential principles of the human mind".

The classical categories (see Cicero and Quintilian) are the demonstrative, to praise or blame; the deliberative, to advise or dissuade; and the forensic, to accuse or defend.

In considering each of these, Campbell believes that not only understanding and memory of the audience must be taken into account, but the orator must as well provide particular attention at stimulating their passions.

Numerous witnesses and no evidence of collusion will supersede all other factors, since the likelihood of testimony outweighs that of Hume's formula for determining the balance of probabilities.

According to Campbell, Hume is wrong to claim that testimony is a weakened type of evidence; it is capable of providing absolute certainty even with the most miraculous event.

Campbell's sermons and lectures provide important insights into the structure of his thought and range of his scholarly activities.