四明十義書 1.90

1.90

以彼簡却所造，唯取能造為境，乃是去其千枝百脈，唯取一根一穴以為所觀。若俱取者，大乖揀境之意也。

Because it sets aside what is produced and takes only that which is capable of producing as the object, this is to remove the thousand branches and hundred pulse-points and take only a single root and a single acupoint as what is contemplated. If one takes both, it greatly contradicts the intention of selecting the object.