Stanley many times in his career made stabs at explaining the imaginative force - the energy - underlying his creativity. Several are included in Andrew Glew's Tate publication Stanley Spencer, Letters and Writings. One of the more accessible was written later in life in a letter to John Rothenstein, the then Director of the Tate Gallery. The inserts and italics are by the author.

Dear John
I am not a naturally able or gifted person at all  [I do not claim to be intellectually academic or a dazzlingly virtuoso painter in the traditional sense.] 
I am only able to do something when I am moved beyond a certain point of my feelings and in a special way. Everything for me (and especially the visible world) has a degree of meaning which I want to see manifested. This can only be done by the degree of belief one has in it, a belief in the infinite [everlasting, transcendental, altruistic, ultimate, eternal] meaning it possesses. ...

At this point in his explanation, Stanley makes a mental leap by asserting that belief in an entity's infinite meaning brings emotional and artistic certainty. He proceeds later in the letter to test out how he arrives at this conclusion, but we can shortcut to it by reference to an earlier explanation in which he states
when I see anything I see everything, and when I can't see one thing, I see absolutely nothing. In other words, the mere perception of a thmg was not necessarily artistically meaningful for him (it wasn't  'By Jove, this is a beautiful world, where's my brush? which would have resulted only in representation : on the contrary, if he could not perceive any meaning in a scene or image, however visually striking, then I see absolutely nothing.) But when he could conceptualise a meaning in a perceived scene or image, or in other words, when he could connect or associate together its qualities and characteristics with others which at first sight may seem disparate, an entity would form of which the meaning could magically emerge as infinite (I see everything.) Then he would be certain that his belief had universal validity.

So now we can continue in his letter : The contemplation of its certainty is peace, and peace, to use a delightfully suitable [ironic] metaphor is the pistol shot that starts off the great race of one's passion to the goal [in his case, its manifestation as an imaginative painting.] This [special or infinite] meaning does not reach me only through the visible world. I notice it in all sorts of contingences and circumstances [such as in my thinking or my emotional reactions  - in my conceptualisation]. It forms in my mind more through means
other than visual ones and it is the conviction that this invisible meaning was a seeable visible thing in this world [and so could be imaged] that is amazing to me.

Thus the word 'imagination' in Stanley's terms refers to a multi-stage dynamic process - astonishing to him that he should be the recipient - whereby not only visible perceptions but also
invisible [abstract] mental comprehensions or concepts could acquire for him certainty [universal 'truth'] through belief in their infiniteness [their eternal verity.] Realisation that a new comprehension was certain and not a passing ephemeral idea would induce in him an emotionally calm state of peace [an 'out-of-this-world' feeling] whereby through contemplation he could convey its invisible special meaning in images which would manifest it [give it pictorial form to make it visible in this world.]

Stanley's functioning is thus the substance of the poetic even if he himself had no verbal poetic facility. In traditional poetic form, the experience is described by the German poet Rilke, in translation by Don Paterson, as : The spirit fallen into quietude knows that what befalls it must be right.

The originality with which Stanley made his abstract thinking palpable in this way consitutes the crux of his art and creativity.