Zohar & Halacha
Rabbi Saul Zucker
Question from a reader: I am a Jew slowly coming out of the Kabbalistic haze. Your article called Tohar HaYichud was one of the most shocking and convincing condemnations of the Kabbalah I have ever seen.
My question for you is, it is clear that Rabbi Yosef Karo based many of his piskei din on the Zohar and other Kabbalists. Most notably, the idea that one should not wear tefilin on Chol Hamoed, the idea of taking one's tefilin off for the musaf prayer on rosh chodesh, or the idea of waiting seven days before being mekadesh the chodesh. Should these psakim be ignored? And should we follow the other opinions on these and related issues?
Rabbi Zucker:  First, it is not true that Rav Yosef Karo based "MANY of his piskei din on the Zohar and other Kabbalists." It is only a few.
 Second, there were many legitimate authorities who did hold from the Zohar, such as the Vilna Gaon. How are we to understand this? The Vilna Gaon must have maintained that the ideas promulgated in the Zohar are very deep, in the sense of the mishnah at the beginning of the second perek of Gemara Chagigah. In his view, the Zohar was written in a way as to hide its true meaning -- thus it is not to be taken literally or superficially. That being the case, there are, in the Vilna Gaon's view, ideas that might manifest themselves in halakhic practice. In this sense, those practices indeed have true ideas at their heart. It is this that Rav Yosef Karo codified. Of course, any practices that do have true ideas at their heart are clearly and unequivocally valid.
 At the same time, it is important to note that at the time that SOME of these issues began, such as not wearing tefillin for Mussaf on Rosh Chodesh, there was a terrible outcry from the standard frum world -- how can you change what has been the practice for thousands of years? In the end, though, Klal Yisroel seems to have accepted the "new" version. (Although it is difficult to believe that the Vilna Gaon, who wore tefillin all day long, took them off for mussaf on Rosh Chodesh). This being the case, IF you have a doubt about the area, you should ask your poseik for a pesak about the specific issue.
I hope this helps somewhat. The main point is that there is a method to halakhic pesak and to learning in general, and as long as this is followed, even if it includes a view based upon something that may be unauthentic (according to many but not to all), then the pesak is legitimate.