'Immaterial' means 'stripped of individual particularities.' It does not quite mean 'abstract': for you might 'immaterialise,' or 'universalise,' let us say, the whole of Nelson's monument, so far as contour and structure go. It is not true, as associationists have taught, that the sight of a house ordinarily brings up to my consciousness impressions of similar buildings which I have seen: all that I am conscious of is the image of this house now before me: but in considering it as a house I consider it apart from the thisness; and so doing I am said to 'purify it of material,' i.e., particular 'conditions.' This explains what St Thomas says presently, "what is inconsistent with intelligibility, is materiality." You cannot understand 'house' in such a way that your intelligence is limited to this house now before you, so that, if you happened to encounter another, you would not know what it was. To understand is to take for a type.
Of God and His Creatures: 2.75