People used to define 'science' as 'the study of the world perceptible through common sense'. This definition is simplistic if not downright näive. Scientifically, one can study the imperceptible world if that world can interact with the perceptible world. The spiritual world is defined by their advocates to be just one such a world! Therefore a claim that there is no science of the spirit is unnecesary contradiction. One should stop at saying that such science isn't well understood yet.
I will elaborate on the physics of something unseen. But this page isn't enough. I will proceed in the 'comments' section.
A visible object is an object whose constituent particles can interact with our electromagnetism. This is because every object, being almost empty, interacts with other objects not by them hitting the 'hard' matter but by electromagnetic attractions an repulsions in the 'empty spaces' in between particles.