Thus it gives form to time in space, "a kind of fourth dimension," ("Essay on the Immediate Data".) or at least it reduces it to nothing more than an abstract scheme of succession, "a stream without bottom or sides, flowing without determinable strength, in an indefinable direction." ("Introduction to Metaphysics".) It requires time to be homogeneous, and every homogeneous medium is space, "for as homogeneity consists here in the absence of any quality, it is not clear how two forms of homogeneity could be distinguished one from the other." ("Essay on the Immediate Data", page 74.) Quite different appears real duration, the duration which is lived. It is pure heterogeneity. It contains a thousand different degrees of tension or relaxation, and its rhythm varies without end. The magic silence of calm nights or the wild disorder of a tempest, the still joy of ecstasy or the tumult of passion unchained, a steep climb towards a difficult truth or a gentle descent from a luminous principle to consequences which easily follow, a moral crisis or a shooting pain, call up intuitions admitting no comparison with one another. We have here no series of moments, but prolonged and interpenetrating phases; their sequence is not a substitution of one point for another, but rather resembles a musical resolution of harmony into harmony. And of this ever-new melody which constitutes our inner life every moment contains a resonance or an echo of past moments. "What are we really, what is our character, except the condensation of the history which we have lived since our birth, even before our birth, since we bring with us our prenatal dispositions? Without doubt we think only with a small part of our past; but it is with our complete past, including our original bias of soul, that we desire, wish, and act." ("Creative Evolution", pages 5-6.) This is what makes our duration irreversible, and its novelty perpetual, for each of the states through which it passes envelops the recollection of all past states. And thus we see, in the end, how, for a being endowed with memory, "existence consists in change, change in ripening, ripening in endless self-creation." ("Creative Evolution", page 8.) With this formula we face the capital problem in which psychology and metaphysics meet, that of liberty. The solution given by Mr Bergson marks one of the culminating points of his philosophy. It is from this summit that he finds light thrown on the riddle of inner being. And it is the centre where all the lines of his research converge.