Complex sentence: It was thanks to Mackenzie that Burns was introduced to Professor Dugald Stewart who had an estate near Catrine on the River Ayr near Mossgiel Farm and was a close friend of Mackenzie, the Burns's family doctor.
Split into simpler sentences: It was thanks to Mackenzie that Burns was introduced to Professor Dugald Stewart. The professor had an estate near Catrine on the River Ayr near Mossgiel Farm and was a close friend of Mackenzie, the Burns's family doctor.