Within some degree of uncertainty in standard fly fishing, like: what is the trout really feeding on, did the grayling see us, is the fly drifting naturally, are the fish eating at a specific depth, etc. we still have a list of possible objective facts to get a grip on. Clues that could determine the correct choice of fly, presentation, etc. In the worst case, a lack of complete evidence may lead us to mount a heavy nymph and search the bottom. In comparison, salmon fishing feels like sailing by sight in the fog.