18th-century philosophers thought and wrote about many things, and many of them were very clever people; but one debate in particular dominated their discourse: what Jerry Fodor recently called 'mind-stuff versus matter stuff':
One might hold that the world isn’t made entirely of matter ... [that] there is also a fundamentally different kind of stuff – mind-stuff, call it – and consciousness resides in that. Notoriously, however, this view has hard problems of its own. For example, if matter-stuff and mind-stuff are of fundamentally different kinds, how are causal relations between them possible? How is it possible that eating should be caused by feeling peckish or feeling peckish by not eating? For this and other reasons, mind-stuff has mostly fallen out of fashion. [LRB, Vol. 29 No. 10 · 24 May 2007; page 9]
Many 18th-century thinkers were motivated by a desire, the grounds and strength of which I don't have time to speculate about here, to retain the revealed religion known as 'Christianity' as part of their way of talking about the world. Descartes suggested that human beings are bodies + souls, the mind-stuff going through a magic router called 'the pineal gland' to enable it to interact with teh matter-stuff. But, for reasons akin to the ones to which Fodor alludes, plenty of people weren't persuaded by that. Other philosophers (Hume, Hartley, Priestly) claimed that there was nothing but matter-stuff, and mind-stuff was just an effect of the way the matter-stuff of the brain operated, not unlike (although this isn't, of course, an analogy any of those gentlemen used) 'speed' emerges from the proper operation of a motorcycle. Hartley believed in God, and used half his most famous book presenting 'proofs' for His existence; but since his account of human beings was entirely material some people accused him of inconsistency (Priestly, who is interesting here predominantly as a diseminator and analyst of Hartley) had an ingenious theory of his own that managed to keep Christianity as 'true' without sacrificing Hartley's materialism. Berkeley approached the problem from the other side, and denied that there was anything called 'matter' -- everything is mind-stuff. But not many people believed him. So there was a breach, between 'soul' and world, that haunted the thinkers of the 18th-century; and it haunted them in part because they worried that the path of truth might compel them to give up 'soul' altogether. This is one reason why Kant proved so influential: he argued, in the Pure Reason critique, that 'mind' and 'world' were not separate entities at all, because key aspects of the world (dimension, causality etc.) were actually the way the soul itself was structured. Coleridge and the second-generation Romantics he inspired took this to be a great healing of the breach. I'm not sure they were right, though.