In all this we find Goethe carefully guarding himself against 'explaining' these atmospheric changes by assuming some kind of purely mechanical cause, such as the accumulation of air-masses over a certain area or the like. Just as little would he permit himself lightly to assume influences of an extra-terrestrial nature, such as those of the moon. Not that he would have had anything against such things, if they had rested on genuine observation. But his own observations, as far as he was able to carry them, told him simply that the atmosphere presses with greater or lesser intensity on the earth in more or less regular rhythms. He was not abandoning the phenomenal sphere, however, when he said that these changes are results of the activity of earthly gravity, or when he concluded from this that barometric variations were caused by variations in the intensity of the field of terrestrial gravity, whereby the earth sometimes drew the atmosphere to it with a stronger, and sometimes with a weaker, pull. He was again not departing from the realm of the phenomenal when he looked round for other indications in nature of such an alternation of drawing in and letting forth of air, and found them in the respiratory processes of animated beings. (To regard the earth as a merely physical structure was impossible for Goethe, for he could have done this only by leaving out of account the life visibly bound up with it.) Accordingly, barometric measurements became for him the sign of a breathing process carried out by the earth. Alongside the alternating phases of contraction and expansion within the atmosphere, Goethe placed the fact that atmospheric density decreases with height. Observation of differences in cloud formation at different levels, of the boundary of snow formation, etc., led him to speak of different 'atmospheres', or of atmospheric circles or spheres, which when undisturbed are arranged concentrically round the earth. Here also he saw, in space, phases of contraction alternating with phases of expansion. * At this point in our discussion it is necessary to introduce another leading concept of Goethean nature-observation, which was for him - as it will be for us - of particular significance for carrying over the Goethean method of research from the organic into the inorganic realm of nature. This is the concept of the ur-phenomenon (Urphänomen). In this latter realm, nature no longer brings forth related phenomena in the ordering proper to them; hence we are obliged to acquire the capacity of penetrating to this ordering by means of our own realistically trained observation and thought. From among the various utterances of Goethe regarding his general conception of the ur-phenomenon, we here select a passage from that part of the historical section of his Theory of Colour where he discusses the method of investigation introduced into science by Bacon. He says: