The sum of the interior angles of a triangle is 180 degrees. If all three angles are greater than 60 degrees, then the angle a+b=c is greater than 180 degrees, which does not conform to the theorem of the sum of interior angles of a triangle, so all three angles are greater than 60 degrees. If this is not true, then the original proposition "In a triangle, at least one internal angle is less than or equal to 60 degrees" is true.