Mantra 4

त्रयो लोका एत एव ।वागेवायं लोको मनोऽन्तरिक्षलोकः प्राणोऽसौ लोकः ॥बृह. १,५.४ ॥

In plain words

These are the three worlds: speech is this world, mind is the middle world, and the breath is that world yonder.