Given their transformative potential, threshold concepts are also likely to be irreversible, i.e. they are difficult to unlearn.

The Academy of Art University, San Francisco, present an excellent one page discussion of Threshold Concepts in which they offer the following simple analogy:
Did you know that there is an arrow embedded in the FedEx logo? Maybe you did. If you didn’t, look for it next time. Once you see it, you will never be able to look at the FedEx logo again without the arrow popping out at you. You will not understand how you ever missed it.
Look for the arrow in the FedEx logo:

Click here if you cannot see the arrow.

BUT, as the Academy of Art University authors themselves stress, this is an amusing but overly simple analogy as a student's mastery of a threshold concept is unlikely to involve instantaneous recognition:
We would argue for the notion of learning as excursive, as a journey or excursion which will have intended direction and outcome but will also acknowledge (and indeed desire) that there will be deviation and unexpected outcomes within the excursion; there will be digression and revisiting (recursion) and possible further points of departure and revised direction. The eventual destination may be reached, or it may be revised. It may be a surprise. It will certainly be the point of embarkation for further excursion.
                                    Ray Land, Glynis Cousin, Erik Meyer & Peter Davies
                                    Threshold concepts and troublesome knowledge (3): implications for course design and evaluation
In short, there is no simple passage in learning from ‘easy’ to ‘difficult’; mastery of a threshold concept often involves messy journeys back, forth and across conceptual terrain
                                    Glynis Cousin, An introduction to threshold concepts
The process of understanding threshold concepts and adjusting one's worldview is often very difficult. Unlike the instant recognition of the arrow in the FedEx logo, understanding a threshold concept is more of a journey of questioning (and perhaps self-doubt), during which students typically go back and forth between using their old lens ("common sense") and their new lens (the threshold concept) until the new concept is comfortable and familiar enough to rely on. Teachers need to be especially supportive of students as they progress through the understanding of a threshold concept. Then and only then can students progress in their understanding of a discipline, and ultimately become contributing members in a field.
                                    The Academy of Art University: Identifying Threshold Concepts
The suggestion that threshold concepts are irreversible is supported by the difficulty that some lecturers find in recognizing threshold concepts that students may encounter in their courses; to quote Cousin again,
One of the difficulties teachers have is that of retracing the journey back to their own days of ‘innocence’, when understandings of threshold concepts escaped them in the early stages of their own learning.
                                    Glynis Cousin, An introduction to threshold concepts

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