The word śāstra is derived from the dhātu, or verbal root, śas. Śas-dhātu pertains to controlling or ruling. A government’s ruling through force or weapons is called śastra. Thus whenever there is ruling, either by weapons or by injunctions, the śas-dhatu is the basic principle. Between śastra (ruling through weapons) and śāstra (ruling through the injunctions of the scriptures), the better is śāstra. Our Vedic scriptures are not ordinary lawbooks of human common sense; they are the statements of factually liberated persons unaffected by the imperfectness of the senses.
Śāstra must be correct always, not sometimes correct and sometimes incorrect. In the Vedic scriptures, the cow is described as a mother. Therefore she is a mother for all time; it is not, as some rascals say, that in the Vedic age she was a mother but she is not in this age. If śāstra is an authority, the cow is a mother always; she was a mother in the Vedic age, and she is a mother in this age also.
If one acts according to the injunctions of śāstra, he is freed from the reactions of sinful activity. For example, the propensities for eating flesh, drinking wine and enjoying sex life are all natural to the conditioned soul.The path of such enjoyment is called pravṛtti-marga. The śāstra says, pravṛttir eṣāṁ bhūtānāṁ nivṛttis tu mahā-phalā: one should not be carried away by the propensities of defective conditioned life; one should be guided by the principles of the śāstras. A child’s propensity is to play all day long, but it is the injunction of the śāstras that the parents should take care to educate him. The śāstras are there just to guide the activities of human society. But because people do not refer to the instructions of śāstras, which are free from defects and imperfections, they are therefore misguided by so-called educated teachers and leaders who are full of the deficiencies of conditioned life.
Ref >> Srila Prabhupada Vani.