Reference for Sanhedrin 112:18

And nevertheless the matter needs a solution, as it is impossible that these righteous ones would be divining. And so it appears to me that the divining that the Torah forbade is when one makes his actions depend upon a sign that logic does not suggest will cause benefit or injury to the thing, such as "bread fell from his mouth," or "a deer blocks him on the way." As these [signs] and those similar to them are from the 'ways of the Amorite.' But if one accepts signs that logically indicate a benefit to the thing or its injury; this is not divining. For all business of the world is like that. As behold, the one who says, "If it rains, I will not go out on the road; but if not, I will go out," is not [practicing] divination. Rather it is the way of the world. And Eliezer and Jonathan were making their actions dependent on things similar to this... And when the Gemara cites [them] regarding the prohibition, this is what it is saying: Any divination of the things the Torah forbids, that logically has no impact - anyone who does not rely upon them like these two relied upon something that was permissible, is not [practicing] divination, and it is not forbidden...