Christian Wolff, Rational Thoughts [German Logic] (1712)


In the preface to this work, also known as the German Logic, Christian Wolff (1679–1754) defines philosophy and its several branches. First published in 1712, Wolff's textbook was reprinted numerous times and aimed to provide a vernacular philosophy that was both accessible and practical. Wolff constructed his philosophy through a complex and thorough system of demonstrations and conclusions. His many works in logic, metaphysics, mathematics, psychology, and natural theology provided the basis for German academic philosophy until Kant.



I. PHILOSOPHY is the Science of all possible things, together with the manner and reason of their possibility.

II. By SCIENCE, I understand, that habit of the understanding, whereby, in a manner not to be refuted, we establish our assertions on irrefragable grounds or principles. What grounds are irrefragable, and what manner is irrefutable, will appear in the course of the following treatise.

III. I call Possible, whatever can be, or whatever implies no contradiction, whether actually existing or not.

IV. As of nothing we can form no conception, so neither of the actual existence of any thing, without a sufficient ground or Reason; which shews, why it rather is than is not; as will appear in its proper place (Metaphysics, § 30, 31.) For the present let it suffice, that the assertion is grounded on experience.

V. A Philosopher therefore ought, not only to know the possibility of a thing, but also assign the reason of that possibility. (§ 1, 2.) It is not enough, that he knows, for instance, the possibility of rain, he must also assign the manner and reason of its possibility.

VI. And hereby the Common or Vulgar Knowledge is distinguished from the philosophical. Though a person, unacquainted with philosophy, may learn much of what is possible from experience, yet he is incapable of assigning the reasons of possibility. From experience, for instance, he may learn the possibility of rain, without being able to shew the manner, or assign the reason of that possibility.

VII. We might therefore be apt to think, that the common or vulgar knowledge is abundantly sufficient for the purposes of life. But as things happen to succeed or answer under certain circumstances only, the common knowledge may oftentimes overlook a circumstance, and then assign, as general, what answers only in a particular case. Thus, for instance, we observe that Compassion is shewn to the miserable, when their misery is made known; and hence we imagine it to be a general truth, that to excite compassion, there needs only a representation of misery. We, in like manner, observe, that rosemary is propagated by slips stuck in the earth, but we would find ourselves greatly disappointed, did we treat all slips of plants, of a close texture, in the same manner. A philosopher, on the contrary, needs never apprehend a misapplication of any one proposition, as he knows the reason why, and time when, a thing shall answer, and never disappoint his expectation (§ 6.). As, that the representation of misery then only excites compassion, when the mind is previously disposed to take a pleasure in the happiness of another: And that a slip will strike root when a knot or joint comes to be covered in the earth, if the slip does not readily wither, and if the rind is with ease penetrated by the striking roots. Such a one is capable also of deducing unknown truths from those already known, and of deriving from his knowledge a pleasure so delicious, as no other can possibly come up to.

VIII. Some may wonder, that we should extend the notion of Philosophy to all possible things, as the wisest must own his share of knowledge to be but scanty, and therefore might think a less presumptuous and more modest definition more adviseable.

IX. I would answer, that it is absolutely more adviseable to frame a definition of Philosophy from its highest possible degree of perfection, rather than from the degree or standard of any man's knowledge of it. As, in this manner, no unnecessary limits will be put to knowledge, which might restrain the mind in exerting its reflective powers, and consequently withhold important discoveries from the world; as was the case when it was thought, that Aristotle had reached the utmost boundaries of human understanding. But every one will rather be excited to make still greater progress, as he is fully sensible much remains still to be discovered; of which our own age, in the case of Mathematics, is an evident proof. It will also teach us humility, not to overrate our presumed attainments, when we find the utmost extent of what we already know, to be but the least part of what remains still unknown. And, in general, it is well known, that things susceptible of degrees are always defined in the general, without restraint to any particular degree. For instance, every man is not temperate in the same degree. In order to define temperance therefore we are not to confine ourselves to this or the other degree thereof, in this or the other person, but, in order to obviate every exception, we extend it to its utmost possible pitch.

X. An attention to ourselves shews, we have a faculty, usually called the Understanding, of perceiving possible things. But, how far this faculty extends, and in what manner we are to use it, either in the discovery of truth by our own reflection, or in passing a rational judgment on the discoveries of others, is not generally so evident. And therefore, in order to know, whether our abilities are fitted for philosophical enquiries, it ought to be our first care, to learn what are the powers of the human understanding, together with their right use and application in the knowledge and search of truth: And that branch of philosophy, which teaches this, is called Logic.

XI. Among possible things, we must admit of one necessary, self-existent Being; otherwise something would be possible, of which no sufficient reason could be assigned, contrary to what was established above, (§ 4.) This self-existent Being we call GOD. Other things, which have the reason of their existence in the self-existent Being, we call Creatures. Now, as the business of philosophy is to assign the reason of the possibility of all things, (§ 5.) the doctrine concerning GOD claims preferably an inquiry, previous to that of the Creatures; an accurate knowledge of which last is to be reduced into, or rather deduced from, their first principles. Though indeed we readily admit we must have a previous common or vulgar knowledge of them, but which we have no occasion to draw from philosophy, as from our early years we have it from experience, (§ 6.) That branch, therefore, of philosophy, which treats concerning GOD, and the origin of the Creature, is called Natural Theology.

XII. The Creatures manifest their activity, either by motion, or by thought: The former, we call bodies, the latter, spiritual beings. Now, as philosophy attempts to assign just reasons for every thing, its province must be, to inquire into the powers and effects, both of the things which act by motion, and of the others, which are self-conscious, by thought. And thus philosophy shews, whatever is possible in the world by the powers, as well of bodies, as of spiritual beings. This last we call Pneumatology; and the other, Physics, or Natural Philosophy.

XIII. That thing within us, which is capable of thought, we call the Soul. Now, as the soul is a spiritual being (§ 12.) and, besides the faculty of Understanding, has also that of Will, on which a great deal, in this world, depends; we are also to shew all that is possible by the will: And to this head belong the Law of Nature, Ethics , Politics, &c.

XIV. As all things, whether bodies or spiritual beings, in some things agree, and in some others disagree or differ; their general agreements, and general disagreements or differences, comprising the general knowledge of things, constitute that branch, which we call Ontology; which last, together with pneumatology and natural theology, forms Metaphysics.

XV. We either proceed no farther in our knowledge than the powers, which produce effects in nature, or we advance one step farther still, namely, in accurately measuring the quantity, both of the powers, and of the effects, in order to shew, that certain powers may produce certain effects. For instance, either we rest satisfied with knowing, that the air, violently compressed, can force water to a considerable height in a fountain, or we attempt accurately to discover the degree of force imparted to the air, when compressed to the half, the third, the fourth, &c. part of its former space, and how many feet high it forces the water in each case. This last degree of knowledge requires the measuring of things, that have quantity. With this view Mathematics were invented. On the different parts of which I have treated in my “German elements of mathematics,” and in the abridgment made therefrom.

XVI. And thus mathematics lead us to the most accurate and perfect knowledge possible to be attained.

XVII. Now, as it is not every one's intention to proceed so far in philosophical knowledge, we incline not in these elements to intermeddle with that higher degree thereof, fully satisfied to have attained to a just knowledge of the powers of nature, and to be thence able to form a judgment of their possible effects.

Source of English translation: Christian Wolff, “Preliminary Discourse Concerning Philosophy,” in Logic, or Rational Thoughts on the Powers of the Human Understanding with their Use and Application in the Knowledge and Search of Truth [German Logic]. London: Printed for L. Hawes, W. Clarke, and R. Collins, 1770, pp. 1–8. Available online at:

Source of original German text: Christian Wolff, Vernünfftige Gedancken von den Kräfften des menschlichen Verstandes & ihrem richtigen Gebrauche in Erkäntnis der Wahrheit. Halle: Renger, 1713; reprinted in Christian Wolff, Gesammelte Werke. Edited and complied by J. Ecole, J. E. Hofmann, M. Thomann and H. W. Arndt, Volume 1. Hildesheim: Georg Olms Verlagsbuchhandlung: 1965, pp. 115–20.

Christian Wolff, Rational Thoughts [German Logic] (1712), published in: German History Intersections, <> [November 30, 2023].