Jamieson, Fausset and Brown on Galatians 6:8

8. Translate, "He that soweth unto his own flesh," with a view to fulfilling its desires. He does not say, "His spirit," as he does say, "His flesh." For in ourselves we are not spiritual, but carnal. The flesh is devoted to selfishness.

corruption--that is, destruction (Php 3:19). Compare as to the deliverance of believers from "corruption" (Ro 8:21). The use of the term "corruption" instead, implies that destruction is not an arbitrary punishment of fleshly-mindedness, but is its natural fruit; the corrupt flesh producing corruption, which is another word for destruction: corruption is the fault, and corruption the punishment (see on 1Co 3:17; 2Pe 2:12). Future life only expands the seed sown here. Men cannot mock God because they can deceive themselves. They who sow tares cannot reap wheat. They alone reap life eternal who sow to the Spirit (Psa 126:6; Pr 11:18; 22:8; Ho 8:7; 10:12; Lu 16:25; Ro 8:11; Jas 5:7).