In this principle, Musashi was thinking about using all the senses – hearing, smell and the other, more peripheral senses – to supplement sight when in combat. It might even apply to not allowing what you are seeing to distract you from other non-visual clues about the what is happening around you. For most applications, that won’t apply to software testing, though of course if your app involves different media channels, that may well apply in its most direct sense.
But for most testers, “perceiving what cannot be seen” will be a rather harder task. We’re back to that old standby, experience.
So many software issues occur when two things interact. Finding that interaction is one of the hardest jobs in testing – “Well, it works on my machine.” There are always hidden outcomes, display issues, and accessibility problems. All of these may fall into the category of “things that cannot be seen”.
Cross-browser testing is one area which may expose “that which cannot be seen”. An app may display differently on one browser to another; one I have seen recently literally has a feature which cannot be seen in one popular browser but is OK in others.
Developing insight is another of these things that comes with experience; and just as I said in an earlier post, it’s never too early to start gaining that experience. Everyone has to start somewhere.
There is also the question of unintended consequences. You will never be able to think through all the possible permutations, but you should try to anticipate some of the more obvious possible ones. The more you think of them, the more new possibilities may occur to you. This is an area where mind mapping or risk storming may be useful techniques.