(Gr. sképsis , speculation, doubt ; sképtesthai, to scrutinize or examine carefully) may mean (1) doubt based on rational grounds, or (2) disbelief based on rational grounds (cf. Balfour, "Defence of Phil. Doubt", p. 296), or (3) a denial of the possibility of attaining truth ; and in any of these senses it may extend to all spheres of human knowledge (Universal Scepticism), or to some particular spheres of the same (Mitigated Scepticism). The third is the strictly philosophical sense of the term Scepticism, which is taken, unless otherwise specified, to be universal. Scepticism is then a systematic denial of the capacity of the human intellect to know anything whatsoever with certainty. It differs from Agnosticism because the latter denies only the possibility of metaphysics and natural theology ; from Positivism in that Positivism denies that we do de facto know anything beyond the laws by which phenomena are related to one another; from Atheism in that the atheist denies only the fact of God's existence, not our capacity for knowing whether He exists.
The great religions of the East are for the most part essentially sceptical. They treat life as one vast illusion, destined some time or other to give place to a state of nescience, or to be absorbed in the life of the Absolute. But their Scepticism is a tone of mind rather than a reasoned philosophical doctrine based upon a critical examination of the human mind or upon a study of the history of human speculation. If we wish for the latter we must seek it among the philosophies of ancient Greece. Among the Greeks the earliest form of philosophical speculation was directed towards an explanation of natural phenomena, and the contradictory theories which were soon evolved by the prolific genius of the Greek mind, inevitably led to Scepticism. Heraclitus, Parmenides, Democritus, Empedocles, Anaxagoras, though differing on other points, one and all came to the conclusion that the senses, whence they had derived the data upon which their theories were built, could not be trusted. Accordingly Protagoras and the Sophists distinguish "appearances" from "reality"; but, finding that no two philosophers could agree as to the nature of the latter, they pronounced reality unknowable. The thorough-going Scepticism which resulted is apparent in the three famous propositions of Gorgias: "Nothing exists"; "If anything did exist it could not be known"; "If it was known, the knowledge of it would be incommunicable."
The first step towards the refutation of this Scepticism was the Socratic doctrine of the concept. There can be no science of the particular, said Socrates. Hence, before any science at all is possible, we must clear up our general notions of things and come to some agreement in regard to definitions. Plato, adopting this attitude, but still holding to the view that the senses can give only dóxa (opinion) and not epistéme ( true knowledge ), worked out an intellectual theory of the universe. Aristotle, who followed, rejected Plato's theory, and proposed a very different one in its place, with the result that another epidemic of Scepticism succeeded. But Aristotle did more than this. He propounded the doctrine of intuition or self-evident truth. All things cannot be proved, he said; yet an infinite regress is impossible. Hence there must be somewhere self-evident principles which are no mere assumptions, but which underlie the structure of human knowledge and are presupposed by the very nature of things (Metaph., 1005 b, 1006 a). This doctrine, later on, was to prove one of the chief forces that checked the destructive onslaught of the Sceptics; for, even if Aristotle's dictum cannot be proved, it none the less states a fact which to many is itself self-evident. It was the Stoics who first took "evidence" as the ultimate criterion of truth. Perceptions, they taught, are valid when they are characterized by enárgeia, i.e. when their objects are manifest, clear, or obvious. Similarly conceptions and judgments are valid when we are conscious that in them there is katálepsis an apprehension of reality. Contemporaneously, however, with Zeno, the founder of Stoicism, lived Pyrrho the Sceptic (d. about 270 B.C.), who, though he admitted that we can know "appearance," denied that we can know anything of the reality that underlies it. Oudèn mâllon -- nothing is more one thing than another. Contradictory statements, therefore, may both be true. A scepticism so radical as this, the Stoics argued, is useless for practical life; and this argument bore fruit. Arcesilaus, founder of the Middle Academy (third century B.C.), though rejecting the Stoic criterion and affirming that nothing could be known for certain, nevertheless admitted that some criterion is needed whereby to direct our actions in practice, and with this in view suggested that we should assent to what is reasonable (tò eúlogon). For "the reasonable" Carneades, who founded the Third Academy (second century B.C.), substituted "the probable": propositions which after careful examination manifest no contradiction, external or internal, are pithané (probable) kaà aperístatos (secure) kaì perideuméne (thoroughly tested) (Sextus Empiricus "Adv. Math.", VII, 166). A subsequent attempt to reconcile conflicting doctrines having proved futile, however, the Academy lapsed into Pyrrhonism. Ænesidemus sums up the traditional arguments of the Sceptics under ten heads, which later on (second century A.D.) were reduced by Sextus Empiricus to five:
In Hume Scepticism finds a new argument derived from the psychology of Locke. A critical examination of human cognition, it was said, reveals the fact that the data of knowledge consist merely of impressions -- distinct, successive, discreet. These the mind connects in various ways, and these ways of connecting things become habitual. Thus the principle of causality, the propositions of arithmetic, geometry, and algebra, physical laws etc., in short all forms of synthesis and relations are subjective in origin. They have no objective validity, and their alleged "necessity" is but a psychological feeling arising from the force of habit. We undoubtedly believe in real things and real causes; but this is merely because we have grown accustomed so to group and connect our mental impressions. The arguments of Pyrrho and other Sceptics are unanswerable, their Scepticism reasonable and well-founded; but in practical life it is too much trouble to think otherwise than we do think, and we could not get on if we did. Kant's answer to Hume was embodied in a philosophy as eminently subjective as that of Hume himself. Consequently it failed, and resulted only in further Scepticism, implicit, if not actually professed. And nowadays physical science, which in Kant's time alone held its own against the inroads of Scepticism, is as thoroughly permeated with it as the rest of our beliefs. One instance must suffice -- that of Mr. A. J. Balfour, who in his "Defense of Philosophic Doubt" seeks to uphold religious belief on the equivocal ground that it is no less certain than scientific theory and method. There is, he says,
A reply to the copious arguments of the Sceptic enumerated above, might take the following line:
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