Hobbes's first area of study was an interest in the physical doctrine of motion and physical momentum. Despite his interest in this phenomenon, he disdained experimental work as in physics. He went on to conceive the system of thought to the elaboration of which he would devote his life. His scheme was first to work out, in a separate treatise, a systematic doctrine of body, showing how physical phenomena were universally explicable in terms of motion, at least as motion or mechanical action was then understood. He then singled out Man from the realm of Nature and plants. Then, in another treatise, he showed what specific bodily motions were involved in the production of the peculiar phenomena of sensation, knowledge, affections and passions whereby Man came into relation with Man. Finally he considered, in his crowning treatise, how Men were moved to enter into society, and argued how this must be regulated if Men were not to fall back into "brutishness and misery". Thus he proposed to unite the separate phenomena of Body, Man, and the State.[citation needed]
Hobbes came home, in 1637, to a country riven with discontent, which disrupted him from the orderly execution of his philosophic plan. However, by the end of the Short Parliament in 1640, he had written a short treatise called The Elements of Law, Natural and Politic. It was not published and only circulated as a manuscript among his acquaintances. A pirated version, however, was published about ten years later. Although it seems that much of The Elements of Law was composed before the sitting of the Short Parliament, there are polemical pieces of the work that clearly mark the influences of the rising political crisis. Nevertheless, many (though not all) elements of Hobbes's political thought were unchanged between The Elements of Law and Leviathan, which demonstrates that the events of the English Civil War had little effect on his contractarian methodology. However, the arguments in Leviathan were modified from The Elements of Law when it came to the necessity of consent in creating political obligation. Namely, Hobbes wrote in The Elements of Law that Patrimonial kingdoms were not necessarily formed by the consent of the governed, while in Leviathan he argued that they were. This was perhaps a reflection either of Hobbes's thoughts about the engagement controversy or of his reaction to treatises published by Patriarchalists, such as Sir Robert Filmer, between 1640 and 1651.[citation needed]
When in November 1640 the Long Parliament succeeded the Short, Hobbes felt that he was in disfavour due to the circulation of his treatise and fled to Paris. He did not return for 11 years. In Paris, he rejoined the coterie around Mersenne and wrote a critique of the Meditations on First Philosophy of Descartes, which was printed as third among the sets of "Objections" appended, with "Replies" from Descartes, in 1641. A different set of remarks on other works by Descartes succeeded only in ending all correspondence between the two.
Hobbes also extended his own works in a way, working on the third section, De Cive, which was finished in November 1641. Although it was initially only circulated privately, it was well received, and included lines of argumentation that were repeated a decade later in Leviathan. He then returned to hard work on the first two sections of his work and published little except a short treatise on optics (Tractatus opticus) included in the collection of scientific tracts published by Mersenne as Cogitata physico-mathematica in 1644. He built a good reputation in philosophic circles and in 1645 was chosen with Descartes, Gilles de Roberval and others to referee the controversy between John Pell and Longomontanus over the problem of squaring the circle.