The 17th century continued to move away from faith-based reasoning and medieval models such as scholasticism. Instead, philosophical systems such as rationalism and empiricism were chosen. Philosophical liberalism also led to an interest in political philosophy.
Thomas Hobbes saw politics as a secular discipline, separate from Aristotelian theology and metaphysics. He had a pessimistic view of humanity as self-centered and competitive rather than benevolent.
His influences were rooted in the deterministic science of the time (Galileo, Newton, Boyle, Hooke ...) and the certainties of mathematics. He visited Galileo and came back convinced that the physical world could be systematized using the new science of dynamics, including the human body and mind and all of civil society.
His masterpiece Leviathan (1651) presented his model of the founding of legitimate states and governments based on theories of social contract. It was written during the Civil War (1642-1651), a struggle for power between Parliament and the King. As a monarchist Hobbes was concerned with demonstrating the need for a strong central authority and avoiding civil conflict. In Leviathan he developed ideas already expressed in his De Cive (1642).
He postulated that life without government would be like a state of nature that would lead to conflict and poverty. To avoid this state of war and insecurity, humans enter into a 'social contract' and establish a civil society. Everyone gives up their natural rights for their protection and abuses of power by authority are the price of peace (although in exceptional cases rebellion may occur). He rejected Lockes' separation of powers arguing that the Sovereign must control the civil, military, judicial and ecclesiastical powers.
His ethics was based on adapting to the situation: if there is no political authority, you have to manage yourself; if there is a political authority, our duty is to obey it. (This is exactly what Hobbes did when the monarchy lost the civil war - save himself by fleeing to Paris.)
John Locke presented the fundamental principles of his epistemology in An Essay on Human Understanding (1690). He argued for empiricism: all our ideas, simple and complex, are based on our experience and sensory stimuli. Our knowledge is, then, severely limited in scope and certainty. We cannot know the inner nature of things, only their behavior and the way it affects us. It is a modified skepticism.
However, it does not mean that everything is unreal. Locke already distinguished between 'primary' and 'secondary' qualities of reality. The primary qualities of an object, such as the solidity and occupation of space exist independently of the perceiver. The secondary ones, like the color, differ according to what he perceives. For example, if we jump in front of a red bus whose primary qualities are solid and take up space, it will cause injury and possibly death. The way the bus appears to us is a controlled hallucination; the bus itself is not.
Locke published his Two Treatises on Civil Government (1690) anonymously to avoid controversy. The first presented arguments against the divine right of kings (root cause of the civil war of the 1640s). The second treaty supported Hobbes' 'social contract' by underlining majority rule. Locke ruled out absolute power and supported the separation of powers.
Baruch Spinoza was a Dutch Jew. In Ethica Ordine Geometrico Demonstrata he disagreed with Descartes stating that mind and body were two names for the same reality. In fact, all reality was a single substance and God and Nature were two names for this same reality of the universe.
He postulated a deterministic pantheism that left no room for free will or spontaneity. We are only free to know that we are determined. (There is a parallel with the predestination of Lutheranism.)
His Ethica demonstrates points in common with stoicism because both philosophies teach how to reach happiness. He disagreed with the stoic idea that reason could dominate emotion. He argued that only one emotion can be dominated by another emotion and that knowledge of passive (not understood) emotions could transform them to active (understandable by reason). (Freud would work with this same hypothesis 200 years later.)
Gottfried Wilhelm Leibniz contributed to the metaphysical debate in his Monadologie (1714), a dualistic idealism programmed by God. The material world would consist of appearances of the real world. (It is similar to the modern idea of the energy composite universe.)
In an attempt to explain Descartes' problem of mind-body interaction, Leibnitz denies causation because everything is prefixed by a God or Almighty Being. (This has a parallel in Spinoza's determinism and Lutheran predestination.)
God is also used in his Principles of Nature and Grace founded on Reason to argue that there was an explanation for everything and an answer to all questions. When asked about God, he replied that His existence was necessary and logical. (Hume and others would argue this claim.)