… The modern doctrine of 'private psychological fields' is the logical result of Hume's doctrine, though it is a result which Hume 'as an agent' refused to accept. This modern doctrine raises a great difficulty in the interpretation of modern science. For all exact observation is made in these private psychological fields. It is then no use talking about instruments and laboratories and physical energy. What is really being observed are narrow bands of colour-sensa in the private psychological space of colour-vision. The impressions of sensation which collectively form this entirely private experience 'arise in the soul from unknown causes.' The spectroscope is a myth, the radiant energy is a myth, the observer's eye is a myth, the observer's brain is a myth, and the observer's record of his experiment on a sheet of paper is a myth. When, some months later, he reads his notes to a learned society, he has a new visual experience of black marks on a white background in a new private psychological field. And again, these experiences arise in his soul 'from unknown causes.' It is merely 'custom' which leads him to connect his earlier with his later experiences.
All exact measurements are, on this theory, observations in such private psychological fields.
But one important fact does emerge from the discussion: that all exact measurements concern perceptions in the mode of presentational immediacy; and that such observations purely concern the systematic geometric forms of the environment, forms defined by projectors from the 'seat' of the strain and irrespective of the actualities which constitute the environment. The contemporary actualities of the world are irrelevant to these observations. All scientific measurements merely concern the systematic real potentiality out of which these actualities arise. This is the meaning of the doctrine that physical science is solely concerned with the mathematical relations of the world.
These mathematical relations belong to the systematic order of extensiveness which characterizes the cosmic epoch in which we live. The societies of enduring objects--electrons, protons, molecules, material bodies--at once sustain that order and arise out of it. The mathematical relations involved in presentational immediacy thus belong equally to the world perceived and to the nature of the percipient. They are, at the same time, public fact and private experience.
The perceptive mode of presentational immediacy is in one sense barren. So far as--apart from symbolic transference--it discloses the contemporary world, that world, thus objectified, is devoid of all elements constitutive of subjective form, elements emotional, appreciative, purposive. The bonds of the objectified nexus only exhibit the definiteness of mathematical relations.
But in another sense this perceptive mode has overwhelming significance. It exhibits that complex of systematic mathematical relations which participate in all the nexus of our cosmic epoch, in the widest meaning of that term. These relations only characterize the epoch by reason of their foundation in the immediate experience of the society of occasions dominating that epoch. Thus we find a special application of the doctrine of the interaction between societies of occasions and the laws of nature. The perceptive mode in presentational immediacy is one of the defining characteristics of the societies which constitute the nexus termed material bodies. Also in some fainter intensity it belongs to the electromagnetic occasions in empty space. From the point of view of a single experient, that mode discloses systematic relations which dominate the environment. But the environment is dominated by these relationships by reason of the experiences of the individual occasions constituting the societies.
Alfred North Whitehead, Process and Reality, Part IV, Chapter V, Section III
