What are the senses?
We noted that one of the significant pieces of evidence that time exists is that in order for thought to be a meaningful concept, it must be dynamic. Without dynamism, an act of cognition or analysis could not transpire because all information content would be static and unchanging. Thought, almost definitionally then, implies a changing Universe.
We could imagine a Universe confined exclusively to this thought-space. That is to say, we could imagine that “my” Universe is composed solely of my thoughts, however they might be said to exist, in a vacuum devoid of any other entity or notion. English lacks the expressive phrasing to truly describe such a situation, but we could think of it as a dark, soundless, space-less realm wherein only my thoughts and memories of past thoughts can be said to exist. Under such a scheme, I would not have any physical manifestation like a body: in fact, notions of space or matter or even abstractions like mathematics would be so foreign that I wonder whether they could even be imagined. Thought itself would be relegated to merely reflecting on past moods and wondering why I appear to be thinking at all; anything beyond that would be utterly futile speculation.
This is manifestly not the case in our Universe, and the implication is that there is more to reality than my thought alone. But then, what else is there, and how might I be aware of it? In broad strokes, we can define the senses to be those faculties which lead me to believe that notions like space and matter and mathematics, for example, exist independent of my thought. I didn’t imagine these notions in isolation or somehow create them out of my thought-space — at least I have no memory of having done so — and I continue to perceive them, often unexpectedly, whether I am actively thinking about them or not.
Perception is stable
That last point, that I continue to perceive these externalities to my thoughts, is important. We could imagine a flurry of continually unprecedented experiences without any pattern or meaning, each moment forever unique in every respect and replaced just as quickly by a totally unlike moment. In one manner of speaking, this actually is the case since information content is always being generated anew through the act of cognition. Hence each moment in time is as a whole unique and unprecedented, even if the component aspects of that moment — the active thought, or the time of day, for example — aren’t necessarily brand new concepts. But for present purposes of argument, this imagined flurry is intended to have no recognizable components as such. Under such a scheme, every atomic component of a moment would itself be unprecedented and would never be repeated at some future time.
Would such a scheme make sense? Under it, thinking would have little significance and the concept of it would nearly break down. Without any similarities or patterns between moments of experience, the only stability would be my train of thought itself; all else would be inherently unpredictable and without cause. It is difficult to see how this would be functionally different from a Universe constrained only to my thought-space. Without anything even in principle capable of being understood or recognized, there would be no way to usefully act except in idle reflection. The nature of these constantly fluctuating externalities would forever be a mystery. At the very least, were we to inhabit such a Universe, then this exercise of determining truth would have now reached its end, that concept being so poorly defined as to be meaningless outside thought alone.
The reader will no doubt agree that we don’t inhabit a patternless Universe. This implies that our awareness of these externalities has at least some stability associated with it, and that certain patterns in our experience do exist across time. That is not to say that perception is infallible. I’ve noticed over time that thinking about certain experiences will sometimes seem to contradict previous experiences and associated conclusions, or at the very least modify them by adding new information and complexity. That mounting complexity is understood through carefully noting our perceptions over time and assembling evidence of behaviour patterns from them. The fact of this complexity and our ability to perceive it motivate the core goal of this exercise: in a Universe of complexities which are capable of being meaningfully analyzed, determining which perceptions more fully reveal or embody stability across time is a nontrivial challenge.
The senses as stable perceptional faculties
Senses, then, are those faculties which to a generally high degree of reliability furnish perception of externalities. There is some debate about it, but this also is a generally accepted definition of senses. Five human senses have been traditionally identified: sight, hearing, smell, taste, and touch. Even evidence that humans have a dedicated mechanism for perceiving the passage of time has been reported. With few exceptions, the majority of humans report to have these senses, which indicates that they are stable enough to reliably identify as well as unique enough to meaningfully differentiate. Understanding the complex function of these senses is the subject of much scientific work and is well beyond the scope of this effort, but for our purposes it is sufficient to note that they are well established.
Significantly, other senses and even extrasensory perception (ESP) have sometimes been claimed to exist. Under our definition, that term is self-contradictory, a result which is pleasingly compatible with the experimental evidence for ESP. Certainly a smaller portion of humans believe in such notions than believe in the traditional senses, and it seems very unlikely that one would trust in ESP while rejecting the existence or reliability of the traditional senses.
Proof by popularity is of course no proof, but it helps to underscore the conclusion. Stability and reliability of perception across time are the hallmarks of any senses. A sense, then, can and should be trusted in proportion to its reliability across time. Trustworthy senses are useful because they help to identify patterns and thereby inform the goals and nature of thought. Care should be taken to firmly establish the consistency of any pattern so identified by bringing as many senses to bear on the pattern as possible and continually analyzing their conclusions as more evidence is gained. The careful reader will note a degree of circularity under this approach, which provides insight to the complimentary natures of thought and sense. Since we define a sense to be a faculty which furnishes perception of anything external to thought, it follows that all else which can be said to exist derives from the thoughtful application of the senses.