Nathan, this question is highly suggestive of an interesting fact -- while there are literally several dozen Systematic Theology works from the Continent in the late 16th, 17th and early 18th centuries, I can barely even account for a small handful of such works from the British Presbyterians of the same period. In truth, the only fully developed High Orthodox system I can account for off the top of my head from a Presbyterian is Edward Leigh's Body of Divinity. Dudley Fenner was an advocate of Presbyterianism and composed a systematic work modeled on Ramist logic, and Cartwright composed a treatise touching on all the principle points of Divinity, but I wouldn't call it a system of Theology. Among the Scotsmen, you have John Brown's lectures, which were mentioned above, and Samuel Rutherford's lectures which were later combined and published under the name Examen Arminianismi. The works of men such as Watson, Boston, Binning and Ridgely, who made expositions, sermons or lectures on the Catechisms approach the concept of a system (some more than others), and there were several such works. Can you think of any Presbyterian systems pre-1800s?
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John Downame? I have not read his system, The Summe of Sacred Divinity. Was he Presbyterian; it seems likely that he was.