I remembered his face but I couldn’t remember his name. Still, it was on the tip of my tongue. However, the more I tried to remember it the more it eluded me.
Our Buddha-nature 佛性 or let’s just call it our true nature, is not hiding from us in some corner of our brain. Our problem is we just can't remember what it is like. We lack direct contact with it enough to say, spontaneously, “Wow, that’s it — oh my god!” And yes it is on the tip of our tongue, so to speak. Still, the more we think about it and try to envision it, the more it eludes us.
Reading about it takes us no farther—we still can’t remember it, directly. We can acknowledge to ourself, for example, that it is unconditioned, mysteriously or spontaneously present, and so on, and even become experts on the subject having read many books. We can call Buddha-nature the sugata-essence, the essence of thought, even though it ever remains unthinkable for us. This might help us to focus more precisely on what it is, based upon the realization of it by others. But, eventually, we have to remember it, personally, (pratyātma). Over time, it may just one day present itself, in its own light. But it won't do so without a lot of hard work on our part to at least get a clear idea of what it is we are seeking.
This realization is not like any other because the observer and the observed become one. They go back to their non-dual origin, in other words. It is not as easy as remembering the person's name we've forgotten but whose face we recognize. Those who embark on such a discovery have to put their whole being into it. They just can hang around the quayside of reality, saving the big voyage for after they retire. They must cross the great ocean which is a dangerous crossing when they are young, not old.