An ideal system (such as a briwckwall filter) is the one which can be described theoretically (mathematically) but cannot be realized practically (physically).
The ideality of the sinc filter, aka the brickwall, results from its frequency-domain definition: an LTI low-pass filter with no ripples in the pass & stop bands and zero transition width.
Having this specification, an ideal lowpass filter, can be described mathematically, but cannot be realized using any practical techniques. One can find the impulse response of an ideal lowpass filter by using any suitable mathematical tools such as the the frequency to time-domain transformation of the given frequency reponse of the filter.
It results that an ideal low-pass filter has an impulse response in the form of a sinc(x) function, which extends from minus to plus infinity in time, a manifestation of its ideality.
Thus, when a sinc filter is to be implemented in practice, it can only be approximated; by truncatation in time for the sinc filter case. The approximation gets better as the length of the truncation increases which, however, is an undesired consequence.
Instead of forcing approximations to ideal sinc filters this way, it is practically more effective to use other techniques, such as weighted windowing, to realize those ideal filters, which follows the conversion of filter characteristics from those of the ideal one to that of the realizable one.