...but only for Dresden Files.
According to the canon, activating and breaking a circle is an act of free will, and that's why this is the mortals' ability. However, one needs to also be careful not to break the circle unintentionally with one's clothes, a shot bullet or even a kicked pebble. This clashes somewhat with the assumption that free will is all about intentions.
So, headcanon: the full mortals use free will the same way as faerie use magic, without really thinking of it, like breathing. It's like the mortals have their personal will fields/auras that can interfere with will-vulnerable constructs, same as wizard auras interfere with technology.
Therefore, sub-headcanon: Thomas, same as other Whites, can break a circle only when he actually intends to, which would be convenient if he used circles more. From his point of view, this has also a downside, though: he can't claim he did it by accident. From Harry's point of view, this is an upside. Especially when they live together.
By the way, this can explain why circles are feline-proof: as far as cats are concerned, they just couldn't give less a damn about whether it's broken or not. (Of course they aren't fully mortal. Come on, they have nine lives.) Zeno's paradox of Dresdenversum about a human trying to break a circle by throwing a cat remains unsolved, though, but if cats owning wizards generally tend to be Mister-like types, the reluctance of researchers in determining the answer on the way of experiment is understandable.
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