Colin Mackenzie (1697/8–1775) was a physician and midwife. Like Goldsmith, Mackenzie received some of his training at Leiden (albeit earlier, in the 1720s). He was a student of the renowned obstetrician William Smellie, mentioned favourably in his Treatise on Midwifery (1764), and he was responsible for some startling advances in obstetric knowledge relating to maternal and foetal blood supplies. Mackenzie's discoveries were claimed by William Hunter, however, and he did not receive due credit until 1780. He ran midwifery courses from the 1750s and also maintained a private lying-in establishment in Southwark until his death in 1775.

Goldsmith may be meeting him in order to advocate on behalf of his nephew William Hodson, recently arrived in London (see Letter 31), perhaps with a view to expanding William's portfolio of medical skills before a departure to India. It is also possible that Mackenzie may have been the ‘friend in Town’ referred to in Letter 36 with the medical and commercial connections to get William a surgeon’s place in India. However, these suppositions are speculative – not least because the extant evidence points towards Mackenzie initiating the contact: his communication to Goldsmith is in the British Library and reads ‘Monday 10 [month illegible] Dr. Mackenzie presents his compliments to Dr. Goldsmith. Begs he will let him know when he can have the pleasure of seeing him.’

The copy-text is the manuscript in the British Library. The date is conjectural and based on the watermark, ‘SP’: the manuscript sheets of Goldsmith's History of the Earth, and Animated Nature, written c. 1772 and also in the British Library, bear the same watermark.