In his 1913 essay "Psychology as the Behaviorist Views It", which is widely credited with (or blamed for!) launching the behaviorist revolt against early introspective psychology, John B. Watson complains
Take the case of sensation. A sensation is defined [by introspective psychologists] in terms of its attributes. One psychologist will state with readiness that the attributes of a visual sensation are quality, extension, duration, and intensity. Another will add clearness. Still another that of order. I doubt if any one psychologist can draw up a set of statements describing what he means by sensation which will be agreed to by three other psychologists of different training.... I firmly believe that two hundred years from now, unless the introspective method is discarded, psychology will still be divided on the question as to whether auditory sensations have the quality of 'extension,' whether intensity is an attribute which can be applied to color, whether there is a difference in 'texture' between image and sensation and upon many hundreds of others of like character (p. 164).
Of course, consciousness studies and introspective psychology are back; and anyone who has delved into the details of scientific or quasi-scientific introspective reports will see the considerable merit in Watson's complaint. And yet it does not follow that there are no facts of the matter to be explored here; maybe it's just hard.
Take the attribute of "clearness". Watson surely has E.B. Titchener in mind here. Titchener characterizes clearness thus:
Clearness... is the attribute which distinguishes the 'focal' from the 'marginal' sensation; it is the attribute whose variation reflects the 'distribution of attention' (1908, p. 26).
Since the term "clearness" has a number of resonances and senses in ordinary language that muddy the issue, Titchener and his students later came to substitute a neologism for it: "attensity". Now, here is the question we must assess to determine if attensity is an attribute of visual sensation: Can two visual experiences be alike in intensity of color, in shape, in resolution of detail, etc., yet differ experientially only in respect of how closely one is attending to them or to their objects -- i.e., in their "attensity"? There are two ways to say no: One might say that degree of attention does not affect visual experience at all, but only later processing, so that my visual experience of this hat before me is exactly the same when I'm attending to it and when I'm not attending to it (assuming all else, such as lighting, angle of eyes, etc., is held constant). Or one might say that degree of attention does affect visual experience but only by means of changing something else, such as the vividness of color or resolution of detail (which of course is another, non-Titchenerian, meaning of "clearness").
Now is this the kind of question we can ever expect an introspective science to answer authoritatively? Or should we join Watson and declare it hopeless? I confess that I myself am torn. I see no reason in principle that we couldn't resolve such matters. Yet the historical divisions of opinion and the muddiness of the answers I expect I would get if I polled people on the matter, and the feeling of lack of progress and the irresolvability of debates between entrenched opponents -- all that gives cause for pessimism....