Whenever a true theory appears, it will be its own evidence.
The word evidence comes from the Latin root meaning “to see”. When we demand evidence of something, that something is either absent by circumstance, or invisible by nature, but we want to “see” it nevertheless.
The scientific method is evidence-based. It presupposes a correspondence between true abstract theories and natural phenomena. This is why theories in physics must be corroborated by experiments and observations. An abstract theory is a model, a map, but the map may be false or inaccurate, and not represent the territory. The scientific method tests and revises our theory constantly so that it approximate nature as much as possible.
In his essay “Nature“, Ralph Waldo Emerson writes, “All science has one aim, namely, to find a theory of nature…Whenever a true theory appears, it will be its own evidence. Its test is, that it will explain all phenomena.”
Note that he says “a true theory appears”. A theory is abstract, how can it “appear”?
I think what Emerson has in mind is incarnation, in the sense that the abstract Supreme Being (Truth, Goodness and Beauty) becomes embodied completely in the concrete. When that happens, it will be its own Evidence, and it would be foolish of anyone to ask for “evidence”, because all “evidence” and all “phenomena” are subsumed under it, and no addition or subtraction is possible.
Emerson, the soul of the Transcendentalist movement, believes in the deity of man, but not the Divinity of Jesus. It is ironic that he gives one of the most succinct explanations of Incarnation, that is, the Truth is its own Evidence, its own Image. As Jesus said, “He who has seen Me has seen the Father”.
In the New Testament, the Evidence is examined:
That which was from the beginning, which we have heard, which we have seen with our eyes, which we have looked upon, and our hands have handled, concerning the Word of life— the life was manifested, and we have seen, and bear witness, and declare to you that eternal life which was with the Father and was manifested to us
1 John 1:1,2
The evidence-based approach is also found in the Old Testament. In a passage foreshadowing the coming of Christ (Deut. 18:15-22), God instructs the Israelites on how to discern between true and false prophets:
And if you say in your heart, ‘How shall we know the word which the Lord has not spoken?’— when a prophet speaks in the name of the Lord, if the thing does not happen or come to pass, that is the thing which the Lord has not spoken; the prophet has spoken it presumptuously; you shall not be afraid of him.
It seems to me at first that God is subjecting divine revelation to the test of evidence by man, and that there is a standard of truth apart from Him. On further reflection, I realize that, since God is the Creator and Lord of nature, his Word must have correspondence and fulfillment in nature, for both originate from Him. This too is a demonstration of the principle of Incarnation.