He had not completed his nineteenth year when he applied to his presbytery to be licensed as a probationer. He was under the legal age, but probably his precocity had made a considerable impression, for the law was evaded under a traditional exception in favour of youths 'of pregnant parts,' and on the 21st July 1799 he became a licentiate. But he did not show much interest in the work of his new calling. Immediately after, he paid a visit to friends in England, in the course of which he preached his first sermon, at Wigan, on 25th August 1799. His eldest brother wrote to his father: 'His mode of delivery is expressive, his language beautiful, and his arguments very forcible and strong.... It is the opinion of those who pretend to be judges that he will shine in the pulpit, but as yet he is rather awkward in his appearance. We, however, are at some pains in adjusting his dress, manner, etc., but he does not seem to pay any great regard to it himself. His mathematical studies appear to occupy more of his time than his religious.'
Returning from England, he spent the next two winters at Edinburgh attending classes at the university. Mathematics, chemistry, natural and moral philosophy, and political economy were the subjects that occupied his attention. To Dr. Robison, Professor of Natural Philosophy, he felt himself under very deep obligations. He had been perplexed by the views which he found in the _Systeme de la Nature_, published under the pseudonym of Mirabeau, but really the work of the Baron von Holbach. That rigid uniformity of natural law which it enforced seemed to point ominously to materialism and atheism. Under Professor Robison's instructions he was led to ponder the remarkable harmony between the human mind and the processes of nature--the wonderful adaptation of the one to the other; and the conclusion was irresistible that this must be due to an intelligent Divine Being who had framed these adaptations. In after years this was the theme of his Bridgewater treatise, and it was one of the corner-stones of his Natural Theology. As to preaching during these Edinburgh studies, it seems to have been almost entirely neglected.
A new situation, however, opened up to him, as assistant to the Rev. Mr. Elliot, minister of Cavers, in Roxburghshire. The duties of this office he discharged for about a year with fair regularity and diligence, but without hard work, and without his showing any lively interest in the objects of the ministry. In the course of his residence there, he learned that when the parish of Kilmany, in Fife, should fall vacant (as it was likely to do, but not just immediately, by the appointment of the incumbent to a chair), he would get the presentation from the University of St. Andrews.
But what interested him much more was his appointment as assistant, for the ensuing session, to the professor of mathematics there. All that concerned the ministry excited but a languid interest, but his literary and scientific ambition was irrepressible. Already it had begun to look towards a mathematical chair. As Mrs. Oliphant remarks, 'The life and energy of a robust young man, full of ambition, eager for achievement, was in all his veins.' As a teacher he kindled the enthusiasm of his students for mathematical science. To himself the demonstrations of geometry were alike complete and beautiful. But he had also a way of associating mathematics with other pursuits, of bringing all manner of side-lights to bear on the study, of finding analogies in this quarter and in that, that greatly increased his popularity as a teacher. As one of his students remarked afterwards, 'Under his extraordinary management the study of mathematics was felt to be hardly less a play of the fancy than a labour of the intellect--the lessons of the day being continually interspersed with applications and illustrations of the most lively nature, so that he secured in a singular manner the confidence and attachment of his pupils.'