I am once again tackling a figure whose work lies on the borders of philosophy and science. I haven't read any one book from him, but instead, I have investigated a selection, the title of which (Priestley on science, philosophy and politics) reflects the variety of the thinker's interests. Because Priestley was also interested of religion, one might say that he played altogether four different roles.
Starting first from the role of a scientific investigator, we may note that Priestely is famous as one of the first people to separate oxygen from air. But Priestley's discovery is an important indication how different it is to do an experiment and to interpret it correctly, as he did not believe he had found a new element – instead, he thought he had managed to purify air from phlogiston, the element that supposedly caused all phenomena involving fire. Priestely quickly noted that this purified air was more invigorating than ordinary air and he also noted that plants appeared to emanate such air.
In addition to dabbling with scientific experiments, Priestly was also a historian of science and worked on an history of electrical studies. Still, perhaps most interesting work of Priestely on the topic of science are his plans for systematising scientific research. Priestly suggests that instead of founding huge societies responsible for all kinds of scientific study, there should be many small societies specialised for a small subfield in science and responsible for collecting funding for that field. In effect, Priestly is suggesting an idea common these days, that universities should concentrate their research on their strong areas.
Priestley's innovative skills were not restricted to mere research politics, but he was, secondly, an eager political thinker. His model of good society was clearly based on United States of America, as he was a stout defender of republicanism and demanded the right of citizens to have an extended field of action, in which state should have no say at all – Priestly goes even so far as to suggest that education should be no business of the state. Furthermore, Priestley speaks vehemently against any privileged class or nobility, except in case of such a class serving the needs of state. And while Priestley did condemn rebellions, he also noted that fight against tyranny was no true rebellion. In line with Priestley's rather Republican ideal of society is his insistence that criminals should be punished hardly, in order to deter anyone from committing a crime. Ironically, Priestly congratulates USA of not being a colonial power and of never falling into a civil war – a mere century would have proven him wrong.
An important part of Priestley's liberal ideal of state is a demand that all religious sects should be tolerated. This is an important element in his third role as religious thinker. Priestley was a unitarian, who thought that Christ did not have a divine nature. Indeed, his philosophical views went even farther away from the traditional religious views. For instance, Priestly denied the possibility of spontaneously working will, but insisted that all actions require some motive – even the act of going against a motive is never unmotivated. Somewhat surprisingly, in addition to basing this thesis on the universal validity of causal principle, Priestley also uses a religious argument – God could not foresee things ahead, if the events of the world would not work in a strictly deterministic manner. Furthermore, he also notes that morals would not work without complete determinism, since a randomly acting person could not be improved in any manner – she could always accidentally act in a completely different manner.
In addition to being a determinist, Priestly was also a materialist. Or, to put it better, he did not believe that there was any wide cleft between material and spiritual phenomena. Matter was not a plenuum or completely solid, but must have been full of emptyness, since it e.g. could be a medium for movement of light. And if matter was really not material in the brute sense of the word, Priestley argued, why could we not say that so-called soul is also made of matter?