Since people rarely conform to laws of physics the ISTP may struggle with interpersonal issues preferring the rationality of scientific experiments to the irrationality of mere mortals with all their foibles, idiosyncrasies and irrationality. The ISTP is intensely practical and the subtleties of emotion will be lost on them. They will also be quite private regarding their own personal lives and walk away from conflict, not from fear but as it is just so unproductive! However this means that others may therefore see them as thick-skinned and uncaring.
Getting close to an ISTP would take time. This is not because they are shy or threat sensitive, it is more that they want to reveal details about themselves at their pace, in their way. Therefore other people may have to go through the ISTP assault course to get genuinely close. The ISTP does not like anything irrational, and emotion is irrational and so they will try to break problems down to constituent parts which may irritate those that ‘just want to be understood.’ The ISTP is a confident introvert, happy with themselves and do not feel the need to show emotion, or even feel emotion as everything gets analysed and stored factually. They genuinely want to help and to solve problems, but these will be solved by dispassionate analysis which may leave those close to them feeling processed, rather than understood.
ISTPs are equally difficult to understand in their need for personal space, which in turn has an impact on their relationships with others. They need to be able to ‘spread out’ both physically and psychologically which generally implies encroaching to some degree on others’ space, especially if they decide that something or someone else's is going to become their next project. (They are generally quite comfortable, however, with being treated the same way they treat others at least in this respect.)