that which presents itself in sensation as a particular. [151] More importance was attached by them to another point—the greater certainty which belongs to thought than to sensation. All the definitions given above point to the immovable strength of conviction as the distinctive feature of knowledge; [152] and of like import is the language attributed to Zeno, [153] comparing simple sensation with an extended finger, assent, as being the first activity of the power of judgment, with a closed hand, conception with the fist, and knowledge with one fist firmly grasped by the other. According to this view, the whole difference between the four processes is one of degree, and depends on the greater or less strength of conviction, on the mental exertion and tension. [154] It is not an absolute difference in kind, but a relative difference, a gradual shading off of one into the other. [(d) The standard of truth.] [(α) Practical need of such a standard.] From these considerations it follows that in the last resort only a relative distinction is left whereby the truth of notions may be tested. Even the general argument for the possibility of knowledge starts with the Stoics by practically taking something for granted. Without failing to urge intellectual objections—and often most pertinent ones [155]—against Scepticism, as was indeed natural, particularly since the time of Chrysippus, [156] the Stoics nevertheless specially took up their stand on one point, which was this, that, unless the knowledge of truth were possible, it would be impossible to act on fixed principles and convictions. [157] Thus, as a last bulwark against doubt, practical needs are appealed to. [(β) Irresistible perceptions the standard of truth.]