Apart from controversial work on Eugenics, Francis Galton seems to be a pioneer in experimental philosophy--when the intellectual distinction between philosophy and psychology was arguably at its most diaphanous. His statistical analysis of individual differences in mental imagery (1880) shows an early interest in testing folk intuitions against the 'expertise' of introspective peers. Then, as now, the ontological status of mental imagery was a topic of inquiry. In an early volume of Mind, he explains:
I desire to define the different degrees of vividness with which different persons have the faculty of recalling familiar scenes under the form of mental pictures, and the peculiarities of the mental visions of different persons. The first questions that I put referred to the illumination, definition and colouring of the mental image, and they were framed as follows...:-
"Before addressing yourself to any of the Questions on the opposite page, think of some definite object--suppose it is your breakfast-table as you sat down to it this morning--and consider carefully the picture that rises before your mind's eye.
1. Illumination--Is the image dim or fairly clear? Is its brightness comparable to that of the actual scene?
2. Definition--Are all the objects pretty well defined at the same time, or is the place of sharpest definition at any one moment more contracted than it is in a real scene?
3. Colouring--Are the colours of the china, of the toast, bread crust, mustard, meat, parsely, or whatever may have been on the table, quite distinct and natural?" [301-302]
Galton first asked his male friends in the scientific world for their responses to his questions because "they were the most likely class of men to give accurate answers concerning this faculty of visualizing, to which novelists and poets continually allude" . He offers no reason why these men would be best able to know mental imagery. Indeed, it turns out that "the great majority... protested that mental imagery was unknown to them, and they looked on me as fanciful and fantastic in supposing that the words 'mental imagery' really expressed what I believed everybody supposed them to mean" . Galton describes his friends as somewhat like colour blind men before knowledge of a perceptive deficiency.
Indeed, a critic said, "these questions presuppose assent to some sort of proposition regarding the 'mind's eye' and the 'images' which it sees... This points to some initial fallacy'... It is only by a figure of speech that I can describe my recollection of a scene as a 'mental image' which I can 'see' with my 'mind's eye'... I do not see it... anymore than a man sees the thousand lines of Sophocles which under due pressure he is ready to repeat. The memory possesses it..."  His friends doubted that references to imagery were anything more than metaphor.
Instead of accepting the word of his peers, Galton persisted. He asked 'the folk' (including women and children) about mental imagery and he found them very forthcoming about their inner experiences. These subjects were surprised that anyone would doubt the veracity of mental images given how vividly they experienced them.
Galton ends up completing a statistical survey on adolescent boys and adult men that yields little data of interest*; nevertheless, his method of seeking empirical results from participants unfamiliar to him might strike a resonance with x-phi researchers.
* Galton found that young boys report a greater vividness of the colour conception than adult men.
Galton, F. (1880) Statistics of Mental Imagery. Mind, 5(19), 301-318