Any mark or attribute, by which is indicated the existence of a particular state or condition, is put in the 3rd case to express this relation.,
जटाभिस्तापसः 'he is an ascetic by (the fact of his having) matted hair.' अपि भवान् कमण्डलुना छात्रमद्राक्षीत् 'your honor might see the student by the fact of his having a kamandalu.' So also छात्रेणोपाध्यायम् 'a teacher by the fact of having students.' शिखया परिव्राजकम् 'a Parivrajaka by a tuft of hair.' But not so here, कमण्डलुपाणिछात्रः ' a student has kamandalu in his hand.' Because here in the compound, कमण्डलुपाणि is hidden the mark.
Why do we say ittha-bhuta? Observe वृक्षं प्रति विद्योतनं.,