@marikitten03 It's not so much about need to, but if you choose to. When reading a stimulus on the LSAT the meaning might come to you immediately from your first glance, but I know from my own personal experience that many times the meaning does not come immediately from first glance. So, to better understand what the stimulus is saying, particularly the conditional relationships, negating or taking the contrapositive helps to clarify what the author is trying to articulate to us.

@ChimziChuku As did I. I recalled shortly after recognizing my mistake that the LSAT writers care deeply about what the author of the passage thinks, not necessarily the scientist's thoughts, and subsequently the specific point the author is trying to make.