bts, there is no way to take a picture, for instance, below the level of the electron, because the instruments we use to observe are electrons. Nevertheless, a great deal can be learned about their structure through mathematical predictions and testing against those predictions. That is, bash the atoms and see what comes out. I am sure I am not telling you anything you don't know already.
Similarly, no-one has taken a tape-measure to a star. Nevertheless, from known constants and triangulation, we can get very precise measurements.
It might be possible, bts, that you are quoting an old adage about the things we "cannot" know, such as how the mind functions, that has been bandied about for decades. But these days we have non-invasive observation tools such as the CT and MRI scans. Subjects can be asked to do various tasks and while we take three-dimensional pictures of what is happening in their brain/mind in real time.