Nyayasutra 335.4

335.4

na sarvo dampatyoḥ saṃyogo garbhādhānahetur dṛśyate tatrāsati karmaṇi na bhavati sati ca bhavatīty anupapanno niyamābhāva iti/ karmanirapekṣeṣu bhūteṣu śarīrotpattihetuṣu niyamaḥ syāt na hy atra kāraṇābhāva iti//65//

Not every intercourse of a husband and wife is seen to be a cause of conception. Since it does not occur when karma is absent, and occurs when [karma] is present, the absence of regularity is [otherwise] inexplicable. If the elements were the causes of the body's origination independently of karma, there would be regularity, for in that case there is no absence of cause.