C.S. Pierce's "response to the anticipated suspicion that he attaches a superstitious or fanciful importance to the number three, and forces divisions to a Procrustean bed of trichotomy."

"I fully admit that there is a not uncommon craze for trichotomies... I am not so afflicted; but I find myself obliged, for truth's sake, to make such a large number of trichotomies that I could not [but] wonder if my readers, especially those of them who are in the way of knowing how common the malady is, should suspect, or even opine, that I am a victim of it. But I am now and here going to convince those who are open to conviction, that it is not so, but that there is a good reason why a thorough student of the subject of this book should be led to make trichotomies, that the nature of the science is such that not only is it to be expected that it should involve real trichotomies, but furthermore, that there is a cause that tends to give this form." (Collected Papers, C.S.Peirce, Principles of Philosophy, 1.568)

"The fact that different ideas are connected is too obvious to be overlooked; yet I have not found any philosopher trying to list or classify all the sources of association. This seems to be worth doing. To me there appear to be only three factors connecting ideas with one another, namely, ŸResemblance, ŸŸContiguity in time or place, and ŸŸCause or ŸEffect.

I don’t think there will be much doubt that our ideas are connected by these factors. ŸA picture naturally leads our thoughts to the thing that is depicted in it; Ÿthe mention of one room naturally introduces remarks or questions about other rooms in the same building; and Ÿif we think of a wound, we can hardly help thinking about the pain that follows it. But it will be hard to prove to anyone’s satisfaction - the reader’s or my own - that this these three are the only sources of association among our ideas. All we can do is to consider a large number of instances where ideas are connected, find in each case what connects them, and eventually develop a really general account of this phenomenon. The more cases we look at, and the more care we employ on them, the more assured we can be that our final list of principles of association is complete." (Enquiry Concerning Human Understanding, Section III, Of the Association of Ideas, David Hume)

"We have three relationships
- one to this bodily shell which envelops us
- one to the Divine cause which is the Source of everything in all things
- and one to our fellow mortals around us." (Marcus Aurelius, Book VIII, 27)

"“Two things fill the mind with ever new and increasing admiration and awe, the more often and steadily we reflect upon them: the starry heavens above me and the moral law within me" (Kant, Critique of Practical Reason, 5:161.33-6)