'A sack' in Genesis 42:25, means a receptacle, and here 'a receptacle' on the natural level, because it is a story about the truths and scientific ideas which are on the natural level. 'The sack' in this passage specifically signifies scientific ideas, because 'a sack' is a container of grain, so the scientific thought is a receptacle of good, and in this specific instance, the good which is from truth.

(References: Arcana Coelestia 5489)