He takes the papers, and says, "I'll read these, but I want a cogent and convincing argument from you, persuading me that the use of mind-altering substances isn't simply a convenient shortcut to a potentially false transcendence."
To Sirius, No offense. The placebo effect is an enhancer even of effective substances, and he's Ravenclaw.
Going on aloud, "Also, I've been convinced," he nods to Sirius, since he's still having issues with actually calling him something out loud, "that even if I'm certain you're no longer possessed and are therefore not running a risk, your own lack of certainty will at the very least hinder your progress. I know you think that this is a good diagnostic, but apparently you're already operating at a disadvantage, trying to undertake the transformation once fully grown, and being on guard against any mind in yours will also keep the animal mind away. I therefore strongly advise reassuring yourself on this point before trying to make contact. Would you be willing to do so, with a less invasive method than legilimency?"