I would like to try to define a conception of emptiness common to all currents of Buddhism, and without using too complex and erudite terms. Here is my attempt:
first of all I would say that conceptually we have to distinguish several levels of realities more or less deep:
1/ there is the sensory world, the sensory perceptions like taste, touch, apple perception, etc. (this is the lowest level of reality);
2/ there is the “solid” world lying behind the sensory perceptions, like the real material apple lying behind my sensory perception of apple (this is the intermediate level of reality);
3/ there is the substantial world lying beyond the solid world, and it is composed of substances like the soul, etc. (this is the deepest level of reality, the ultimate level)
Now, here is my definition of emptiness: it is the idea that " denies the existence of this or that substance, and denies at least of the existence of the substantial individual Self".
From this definition, we can derive (at least) two other sub-concepts of emptiness: restrictive emptiness and universal emptiness.
Restrictive emptiness is the idea that “there is no substantial individual Self, but there are other substances”.
And universal emptiness is the idea that “not only is there no substantial individual Self, but there is absolutely no substance”.
So, for example, the Sarvastivadins will say that such and such a substance exists, but they will also say that such and such a substance does not exist (namely, the individual Self). This makes them accept the idea of emptiness in their own way, because they at least deny the existence of a certain substance. The Sarvastivadins thus partly accept the substantial world. But they also partly deny it, because they deny the individual substantial Self. So they do have emptiness. More precisely, restrictive emptiness.
And for example, the Madhyamaka will deny the existence of absolutely every substance imaginable. So of course the Mahayana also has the idea of emptiness in its own way, because it denies substances. Here, Madhyamaka totally denies the substantial world. More precisely, Madhyamaka has therefore universal emptiness.
Moreover, with my classification of the three levels of reality, we notice that :
• some currents (Sarvāstivāda) accept the existence of the three levels: sensible, solid, substantial.
• some currents (Madhyamaka) accept only 2 levels: sensible and solid, and deny the substantial.
• some currents (Yogacara) accept only 1 level: sensible, and deny the solid and substantial.
What do you think about this? Am I too naive in my definition of “substance”? I know it is rather vague, but I did it on purpose to avoid falling into a hyper-technical erudition that would make my brain smoke.
Thank you for your patience