Pure being is so indeterminate that it is not even being and so is nothing. This is one gloss on the basic Hegelian idea at the beginning of the Science of Logic. But what does it mean?
To achieve a correct understanding of this equation of pure being and nothing, we need to consider if it is required that we screen out properties that are indeterminate in the sense of not distinguishing between determinate beings. If everything were necessarily spatiotemporally located, for example, then being spatiotemporally located would be an indeterminate property in this sense. Yet something spatiotemporally located is not nothing, so we must be careful if we are going to suppose that the equation of pure being and nothingness rests on indeterminacy of this sort.
Why would we want to avoid this supposition? Well, insofar as we are capable of contrasting relative and absolute nothingness, it seems clear that the latter is incompatible with this sort of indeterminacy. So we have to get rid of this indeterminacy if we want to reach all the way through to absolute nothingness. And, if we take away all the indeterminate properties of pure being, as well as all the determinate ones, then we arguably don’t have a subject of predication at all, and that would seem equivalent to nothing in this absolute sense; or at least it would be if there were a workable notion of negation floating free of any predication.
Again, if we still have some indeterminate properties, e.g. self-identity, then we have probably not done enough to purify the concept of pure being so that it is capable of validating the equation with absolute nothingness. (That is, we have not done enough to destroy any means of distinguishing it from the latter.)
What prevents us from succeeding along these lines, then? The only barrier would be the inconceivability of absolute nothingness itself, which arises from the apparent inability to express any sort of negation without having that negation mean that something (and hence not nothing) is the case.
On the other hand, how do things stand if we adhere closely to the view that absolute nothingness is inconceivable? Then any negation of pure being would at least require that ‘being’ be taken equivocally, i.e. as something that could be wholly replaced by something else (and not by absolutely nothing). Yet, in this case, the being in question would clearly not be pure, because it would be determined against the being that negates it. Basically, as long as we take being to be an individual, it’s not going to be possible to completely purify it, even if we give up the goal of removing all its indeterminate properties (so as to equate it with absolute nothingness). Such a being would still be determined against every determinate thing, and thus be itself determinate (and so impure).
So to summarise: the purest being is impure and the most absolute nothing is relative.