Heisenberg's uncertainty principle states that it is impossible to determine precisely the exact position and momentum of a particle; the product of the "uncertainty values" of both is greater than some universal (Planck's) constant. The more precisely you determine the position, the more uncertain you are about the precise momentum, and vice versa. In the case of Avineri, he should know that "trend towards" something is the "momentum". Israel is far from being exactly fascist, but the TREND is towards fascism. And the stronger the trend (i.e., the better the momentum is known), the less precise can the exact position be stated.
Firebomb thrown during demonstration in northern Israeli town of Umm al Fahm (Haaretz)
from the article: More than a few fringe extremists threaten Israeli democracy