RATIONALISM (from Lat. rationalis, pertaining to reason, ratio), a term employed both in philosophy and in theology for any system which sets up human reason as the final criterion and chief source of knowledge. Such systems are opposed to all doctrines which rest solely or ultimately upon external authority; the individual must investigate everything for himself and abandon any position the validity of which cannot be rationally demonstrated. The rationalist spirit is, of course, coeval with human evolution; religion itself began with a rational attempt to maintain amicable relations with unknown powers, and each one of the dead religions succumbed before the development of rationalist inquiry into its premises.
But the term has acquired more special connotations in modern thought. In its commonest use it is applied to all who decline to accept the authority of the Bible as the infallible record of a divine revelation, and is practically synonymous with freethinking. This type. of rationalism is based largely upon the results of modern historical and archaeological investigation. The story of the Creation in the book of Genesis is shown, from the point of view of chronology, to be a poetic or symbolic account by the discovery of civilizations of much greater antiguity. Again, the study of comparative religion (e.g. the study of the Deluge (q.v.), showing as it does that similar stories are to be found in primitive literature, both oriental and other) has placed the Bible in close relation with other ancient literature.
The Bible, especially the Old Testament, is thus regarded even by orthodox Christians from a rationalist standpoint, very different from that of the early and medieval Church. Rationalism within the Christian Church differs, however, from that which is commonly understood by the term, inasmuch as it accepts as revealed the fundamental facts of its creed. Thoroughgoing rationalism, on the other hand, either categorically denies that the supernatural or the infinite—whether it exist or not—can be the object of human knowledge (see Agnosticism), or else, in the mouth of a single person, states that he at least has no such knowledge.
In addition to the difficulties presented by the Bible as an historical record, and the literary problems which textual and other critics have investigated, the modern freethinker denies that the Christianity of the New Testament or its interpretation by modern theologians affords a coherent theory of human life and duty. Apart from the general use of the term for a particular attitude towards religion, two more technical uses require notice: (i) the purely philosophical, (ii) the theological.
(i) Philosophical rationalism is that theory of knowledge which maintains that reason is in and by itself a source of knowledge, and that knowledge so derived has superior authority over knowledge acquired through sensation. This view is opposed to the various systems which regard the mind as a tabula rasa (blank tablet) in which the outside world as it were imprints itself through the senses. The opposition between rationalism and sensationalism is, however, rarely so simple and direct, inasmuch as many thinkers (e.g. Locke) have admitted both sensation and reflection. Such philosophies are called rationalist or sensationalist according as they lay emphasis specially on the function of reason or that of the senses.
More generally, philosophic rationalism is opposed to empirical theories of knowledge, inasmuch as it regards all true knowledge as deriving deductively from fundamental elementary concepts. This attitude may be studied in Descartes, Leibnitz and Wolff. It is based on Descartes' fundamental principle that knowledge must be clear, and seeks to give to philosophy the certainty and demonstrative character of mathematics, from the a priori principle of which all its claims are derived. The attack made by David Hume on the causal relation led directly to the new rationalism of Kant, who argued that it was wrong to regard thought as mere analysis. A priori concepts there are, but if they are to lead to the amplification of knowledge, they must be brought into relation with empirical data.
(ii) The term "rationalism" in the narrow theological sense is specially used of the doctrines held by a school of German theologians and Biblical scholars which was prominent roughly between 1740 and 1836. This rationalism within the Church was a theological manifestation of the intellectual movement known as the Enlightenment (Aufklärung), and must be studied in close connexion with the purely philosophical rationalism already discussed. It owed much to the English deists, to the Pietistic movement, and to the French esprits forts who had already made a vigorous attack on the supernatural origin of the Scriptures. The crux of the difficulty was the doctrine of the supernatural, the relation between revealed and natural religion.
The first great rationalist leader was J. S. Semler (q.v.), who held that true religion springs from the individual soul, and attacked the authority of the Bible in a comprehensive spirit of criticism. He ultimately reached a point at which the Bible became for him simply one of many ancient documents. At the same time he did not impugn the authority of the Church, which he regarded as useful in maintaining external unity. Among those who followed in Semler's path were Gruner Ernesti, J. D. Michaelis, Griesbach, J. G. Eichhorn. This spirit was exhibited on the philosophical side by Kant who in his Die Religion innerhalb der Grenzen der blossen Vernunft (1793) set forth his doctrine of rational morality (Vernunftglauben) as the only true religion.
These two great rationalist movements, the critical and the philosophical, ultimately led to, or were accompanied by, the gradual reduction of religion to a system of morals based at the most on two or three fundamental religious principles. This is the rationalism known as rationalismus vulgaris, the period of which is practically from 1800 to 1833. Among its exponents were Wegscheider, Bretschneider and H. E. G. Paulus (qq.v.). The general attitude of German theology, however, became gradually more and more hostile, and the works of Schleiermacher, though in a sense themselves rationalist, renewed the general desire for a positive Christianity.
Hase's Hutterus Redivivus, an exposition of orthodoxy in the light of modern development, called forth a final exposition of the rationalist position by Rohr. From that time the school as such ceased to have a real existence, though the results of its work are traceable more or less in all modern Biblical criticism, and its influence upon the attitude of modern theologians and Biblical critics can scarcely be overestimated.
See Ständlin, Geschichte des Rationalismus (Göttingen, 1826); Hase, Theologische Streitschriften in Gesammelte Werke, viii. (1892); Rückert, Der Rationalismus (1859); Tholuck, Vorgeschichte des Rat. (1853-1861) and Geschichte des Rat. (1865); Ritschl, Christ. Lehre von der Rechtfertigung, &c. (1870), vol. i.; Benn, History of Rationalism (1906). See also histories of philosophy and theology in the 19th century, and the valuable article s.v. by O. Kirn in Herzog-Hauck, Realencyk. xvi. (1905).