Rationalism and empiricism have had many definitions, most concerned with specific schools of philosophy or groups of philosophers in particular countries, such as Germany. In general rationalism is the predominant school of thought in the multi-national, cross-cultural Age of reason, which began in the century straddling 1600 as a conventional date, empiricism is the reliance on sensory data gathered in experimentation by scientists of any country, who, in the Age of Reason were rationalists. An early professed empiricist, Thomas Hobbes, known as an eccentric denizen of the court of Charles II of England (an "old bear"), published in 1651 Leviathan, a political treatise written during the English civil war, containing an early manifesto in English of rationalism.
"The Latines called Accounts of mony Rationes ... and thence it seems to proceed that they extended the word Ratio, to the faculty of Reckoning in all other things....When a man reasoneth hee does nothing else but conceive a summe totall ... For Reason ... is nothing but Reckoning ... of the consequences of generall names agreed upon, for the marking and signifying of our thoughts ...."
In Hobbes reasoning is the right process of drawing conclusions from definitions (the "names agreed upon"). He goes on to define error as self-contradiction of definition ("an absurdity, or senselesse Speech") or conclusions that do not follow the definitions on which they are supposed to be based. Science, on the other hand, is the outcome of "right reasoning," which is based on "natural sense and imagination", a kind of sensitivity to nature, as "nature it selfe cannot erre."
Having chosen his ground carefully Hobbes launches an epistemological attack on metaphysics. The academic philosophers had arrived at the Theory of Matter and Form from consideration of certain natural paradoxes subsumed under the general heading of the Unity Problem. For example, a body appears to be one thing and yet it is distributed into many parts. Which is it, one or many? Aristotle had arrived at the real distinction between matter and form, metaphysical components whose interpenetration produces the paradox. The whole unity comes from the substantial form and the distribution into parts from the matter. Inhering in the parts giving them really distinct unities are the accidental forms. The unity of the whole being is actuated by another really distinct principle, the existence.
If nature cannot err, then there are no paradoxes in it; to Hobbes, the paradox is a form of the absurd, which is inconsistency: "Natural sense and imagination, are not subject to absurdity" and "For error is but a deception ... But when we make a generall assertion, unlesse it be a true one, the possibility of it is inconceivable. And words whereby we conceive nothing but the sound, are those we call Absurd ...." Among Hobbes examples are "round quadrangle", "immaterial substance", "free subject." Of the scholastics he says:
"Yet they will have us beleeve, that by the Almighty power of God, one body may be at one and the same time in many places [the problem of the universals]; and many bodies at one and the same time in one place [the whole and the parts]; ... And these are but a small part of the Incongruencies they are forced to, from their disputing philosophically, instead of admiring, and adoring of the Divine and Incomprehensible Nature ...."
The real distinction between essence and existence, and that between form and matter, which served for so long as the basis of metaphysics, Hobbes identifies as "the Error of Separated Essences." The words "Is, or Bee, or Are, and the like" add no meaning to an argument nor do derived words such as "Entity, Essence, Essentially, Essentiality", which "are the names of nothing" but are mere "Signes" connecting "one name or attribute to another: as when we say, "a man is a living body", we mean not that the man is one thing, the living body another, and the is, or being a third: but that the man, and the living body, is the same thing; ..." Metaphysiques, Hobbes says, is "far from the possibility of being understood" and is "repugnant to natural reason."
Being to Hobbes (and the other empiricists) is the physical universe:
The world, (I mean ... the Universe, that is, the whole masse of all things that are) is corporeall, that is to say, Body; and hath the dimension of magnitude, namely, Length, Bredth and Depth: also every part of Body, is likewise Body ... and consequently every part of the Universe is Body, and that which is not Body, is no part of the Universe: and because the Universe is all, that which is no part of it is nothing; and consequently no where."
Hobbes' view is representative of his tradition. As Aristotle offered the categories and the act of existence, and Aquinas the analogy of being, the rationalists also had their own system, the great chain of being, an interlocking hierarchy of beings from God to dust.