He said the secret of Aikido was to do with the natural universal movements and he also said the secret of Aikido was not how to move your feet but how to move your mind for he teaches nonviolence. He said the secret of was thus many things. He said the secret when taking on tenryu the sumo was the circle.
So it's best when speaking English not to use the term secret at all unless you're trying to sell newspapers because it sounds just like a headline attention grabbing statement. Best use the proper term rather than the mystical intriguing term and that would be principle.
Then put that in context by saying it is a principle rather than the principle.
And every single one of those comes from classical Chinese sources, I've discussed some of them before.
I used the term because he did - and attention grabbing or not, in Japanese or English it indicates a thing of some importance.
There's a Japanese word for "principle" and he didn't use it in this case. He did in many others, so he clearly understood the difference.
Third times the charm, now I'm really out.