Hobbes's assertion of a plenum is his response to a years-long philosophical debate against vacuism, or the theory that the universe is largely devoid of matter. Still, though Hobbes claims (as we will see in the next section) that philosophical truth must be deduced from shared definitions, he does not here indicate that his own fundamental first principle of the plenum is generally accepted or agreed upon; Hobbes acts as his own arbitrator and judge of first principles. His philosophical project manages to remain logically consistent only by recursively validating these first principles in later chapters. To dispute the truth value of Hobbes's unspoken claim that nature is a plenum is not necessarily to dispute the entire edifice that is Leviathan, for Hobbes argues from common experience at several points. However, so tightly structured is the text, with one step leading to the next step, with one layer founding the succeeding layer, that--as with a house of cards--tearing out the bottom tier would threaten to topple the upper stories.
Of course, as we will see in the next section, Hobbes is proposing an epistemological system whose foundations need not be universally true as long as they are conventionally agreed upon for the sake of attaining civil peace. This factor alone prevented Hobbes's vacuist contemporaries from dismissing his project on the basis of its controversial first principles.