Obviously, the difference between Ni and Ti is that the former is a Perceiving function and the latter is a Judging function, but nonetheless, perhaps because both are involved in determining the truth, understanding the exact differences can be confusing.
Ni searches for the right answer, while Ti checks and evaluates the answers based on what we do know. Ni determines what could be true, while Ti determines what *is* true.
Einstein (INTP) who was Ti-dominant famously said that if he had 1 hour to solve the world’s problems that he would spend the first 55 minutes finding the right question to ask because then the solution would be obvious. I think Tesla (INTJ) would have spent 5 minutes developing a few possible theories, with the rest of the time testing them in a lab. For the Ni user the proof is in the pudding or in the invention and as per a Personality Hacker article on INTP vs INTJ, for Einstein the proof would be in the math.
The direction is different with Ni pointing from inside into the external world and vice versa for Ti. Ni takes an array of internal impressions (possible answers) that are filtered out and presented to the world with Te or Fe. These Judging functions identify and throw out the ones that are garbage. Perceiving functions tell you what something is and Judging functions tell you what it’s worth.
Ti on the other hand is a deductive process that takes external data (people’s behavior, definitions, facts, or known theories) and deduces whether the current situation is consistent with other known things. It can also be used to clean data and to make sure there is no garbage involved in the input data. It is a checking function that is asking, “can it be so? Does it make sense? Is it consistent? ” It is *not* an idea-generating process, it is an idea-evaluating process. Unlike Ni, it is checking and not hypothesizing, and it is confusing to tell the difference because they have the same objective of getting to the underlying principle but at different stages of the discovery process.
In its raw form, Ni just searches for possible answers, and doesn’t actually provide the “universal principle ” like I said until Te has checked whether the idea is viable. Te is an “inductive” process that checks what option works and because it is based in heuristics and criteria is much faster at evaluating options than its Ti counterpart, which is “deductive” and therefore also more accurate.
Ultimately, Ti says “if this is what I know, then what are the questions that I can answer?” and Ni does it backwards saying “If this is the question, then what are some possible answers?”.