Abstract: A new interpretation of the MHD is given in this paper. According to it, most of the people are supposed to represent the MHD by mental image and obey the law of parsimony in their representation; Based on the two of these, people can only represent the MHD by dynamic mental image in the first stage of the problem, i.e., people can only imagine that the car appears randomly among the three doors; And at the second stage of the problem, people can also only imagine that the car appears randomly between the two doors which were left for the same reasons; though, they should have imagined that the car appears between the two doors underlying the ratio of 1:2. That is why the reasoning illusion exists.

Key words: MHD, the law of parsimony, mental image representation