Because our experience of the world is mediated by our sensation of it, reality, or objective nature, does not necessarily provide universally satisfying definitions by itself. Hobbes writes, "For though the nature of that we conceive, be the same; yet the diversity of our reception of it, in respect of different constitutions of body, and prejudices of opinion, gives everything a tincture of our different passions. And therefore in reasoning, a man must take heed of words; which besides the signification of what we imagine their nature, have a signification also of the nature, disposition, and interest of the speaker."
Hobbes suggests that the observation of nature and the sensation of the material world is always affected by the individual character of the observer, and therefore experience of natural phenomena and the perception of reality do not constitute an adequate basis upon which to ground philosophically true conclusions to a train of thought.
As long as there persist differences in experience, which in turn correspond to differences in meaning, true certainty cannot be achieved. We cannot simply turn to nature as a basis of truth, for objective nature--nature in itself--is inaccessible to us, always filtered through a screen of subjectivity. Thus, Hobbes decides, there must be some governing body, unanimously recognized, appointed to settle the definitions of words and first principles: "But no one mans Reason, nor the Reason of any one number of men, makes the certaintie; no more than an account is therefore well cast up, because a great many men have unanimously approved it. And therefore, as when there is a controversy in an account, the parties must by their own accord, set up for right Reason, the Reason of some Arbitrator, or Judge, to whose sentence they will both stand, or their controversy must either come to blows, or be undecided, for want of a right Reason constituted by Nature; and so it is also in all debates of what kind soever."
Hobbes points out that there is no "right Reason constituted by Nature," again noting the ineffectiveness of employing nature as the foundation of knowledge. He also points out that the judge who will settle definitions--the definitions upon which everyone agrees to agree--is appointed by the participants "by their own accord." It is this judge (eventually revealed as "the sovereign" in Chapter 18) who then becomes the needed foundation of all knowledge.
Thus, definitions are agreed upon because they are determined by a judge whose decisions everyone has agreed to uphold. With this method for securing the foundation of truth, Hobbes then elaborates his complete program for a reform of philosophy and the institution of a science that will provide secure knowledge and put an end to disagreement and social discord.
The process of science, Hobbes says, is reason, and "Reason . . . is nothing but Reckoning (that is, Adding and Subtracting) of the Consequences of generall names agreed upon." Each step of the reasoning process must itself be secure in its claims, like a carefully wrought object of perfect integrity: "The Use and End of Reason, is not the finding of the summe, and truth of one, or a few consequences, remote from the first definitions, and settled signification of names; but to begin at these; and proceed from one consequence to another. For there can be no certainty of the last Conclusion, without a certainty of all those Affirmation and Negations, on which it was grounded, and inferred." From this mathematical process of philosophical reasoning, with its language of arithmetic and its geometric accretion of consequences and conclusions, one arrives at proper science: "Reason is . . . attayned by Industry; first in apt imposing of Names; and secondly by getting a good and orderly Method in proceeding from the Elements, which are Names, to Assertions made by Connexion of one of them to another; and so to Syllogismes, which are the Connexions of one Assertion to another, till we come to a knowledge of all the Consequences of names appertaining to the subject in hand; and that is it, men call SCIENCE."