While admitting the complexity and sophistication of the Jain doctrine, Padmanabh Jaini compares it with that of Hindu doctrine of rebirth and points out that the Jain seers are silent on the exact moment and mode of rebirth, that is, the re-entry of soul in womb after the death. The concept of "nitya-nigoda", which states that there are certain categories of souls who have always been "nigodas", is also criticized. According to Jainism, "nigodas" are lowest form of extremely microscopic beings having momentary life spans, living in colonies and pervading the entire universe. According to Jaini, the entire concept of "nitya-nigoda" undermines the concept of karma, as these beings clearly would not have had prior opportunity to perform any karmically meaningful actions.