In plain words
Stainless, not dull of mind, pure in understanding, being the stainless Self: through that pure seeing he regards all that he sees as pure, and he sees it so. Consider this.
Word for word
Stainless, not dull in understanding, pure in intellect and as the stainless Self, one contemplates through pure understanding, ‘All that is seen is pure’, and sees accordingly. Consider this.