Specific objects

Donald Judd redefined sculpture to be about creating space, rather than simply occupying it.
He was interested in three things - space, material and colour.
His insight was that pre-modern sculpture simply occupied space. In opposition to this, he defined the concept of Specific Objects. His work was not mean to occupy space, but define space itself. This is why his work was so often serials or sequences. The gaps in the sequence are the most interesting bit.