More like non-Gallifreyans are more suspect than Gallifreyans...or aliens I know very well. [He adds, thinking of Leela. He trusts Leela completely, alien or not.]

And, yes, I know it isn't virtuous of me [He's been told off by Leela and Romana enough times], but I know my people. I understand them. If a Gallifreyan is up to no good, chances are I know what motivates them and how they think, how they plan. Dangerous as a renegade Time Lord can be, they aren't unknown. Aliens are more difficult to predict. I feel I can never be sure what they want.