 Von Neumann thus needs to show that the final predictions for what we consciously observe are insensitive to how far along such a ‘measurement chain’ one chooses to continue applying intervention \(\mathbf{2}\), thus ensuring that every step in the process of observation can be described purely in physical terms. In von Neumann’s example of a measurement of temperature, we need not apply intervention \(\mathbf{1}\) to the system itself, but may apply it to the thermometer, or to the retina in the eye, or to the optic nerve, or anywhere else within the physical realm between the system and the ‘abstract ego’ of the observer. By the same token, however, we can (much more practically!) apply it also directly to the measured system. 