2.5 It is evident that the conception of the instant of time as an ultimate entity is the source of all our difficulties of explanation. If there are such ultimate entities, instantaneous nature is an ultimate fact. Our perception of time is as a duration, and these instants have only been introduced by reason of a supposed necessity of thought. In fact absolute time is just as much a metaphysical monstrosity as absolute space. The way out of the perplexities, as to the ultimate data of science in terms of which physical explanation is ultimately to be expressed, is to express the essential scientific concepts of time, space and material as issuing from fundamental relations between events and from recognitions of the characters of events. These relations of events are those immediate deliverances of observation which are referred to when we say that events are spread through time and space. 3. Perception. 3.1 The conception of one universal nature embracing the fragmentary perceptions of events by one percipient and the many perceptions by diverse percipients is surrounded with difficulties. In the first place there is what we will call the 'Berkeleyan Dilemma' which crudely and shortly may be stated thus: Perceptions are in the mind and universal nature is out of the mind, and thus the conception of universal nature can have no relevance to our perceptual life. This is not how Berkeley stated his criticism of materialism; he was thinking of substance and matter. But this variation is a detail and his criticism is fatal to any of the traditional types of 'mind-watching-things' philosophy, even if those things be events and not substance or material. His criticisms range through every type of sense-perception, though in particular he concentrates on Vision. 3.2 "Euphranor.[2] Tell me, Alciphron, can you discern the doors, windows, and battlements of that same castle? Alciphron. I cannot. At this distance it seems only a small round tower. Euph. But I, who have been at it, know that it is no small round tower, but a large square building with battlements and turrets, which it seems you do not see. Alc. What will you infer from thence? Euph. I would infer that the very object which you strictly and properly perceive by sight is not that thing which is several miles distant. Alc. Why so? Euph. Because a little round object is one thing, and a great square object is another. Is it not so? Alc. I cannot deny it. Euph. Tell me, is not the visible appearance alone the proper object of sight? Alc. It is. What think you now (said Euphranor, pointing towards the heavens) of the visible appearance of yonder planet? Is it not a round luminous flat, no bigger than a six-pence? Alc. What then? Euph. Tell me then, what you think of the planet itself? Do you not conceive it to be a vast opaque globe, with several unequal risings and valleys? Alc. I do.