The soul is the part of us that transmigrates after our life ends. It is the part of us that never dies; it is what gives the rest of our body vitality and movement. The soul determines the fate, consciousness, and presence of the body. When the body loses life, we say than Joe has passed away or passed on, but what is it that has ‘passed on/ away?’ It is that nonperishable quality of ourselves that migrates towards fulfillment of its next journey. The body perishes: decays in the grave, burns to ashes, or sits in the morgue so while this tangible and temporary part of us dies that immortal and omnipotent part of us passes on.
The soul ignites the body: it brings the body into existence, brings consciousness to the body, and allows us to live. Without the soul there is no ‘I.’ Have you ever wondered what makes you you? Your hands, your heart, your body; a body is just a body, a hand is just a hand, and a heart is just a heart. What makes it uniquely yours is the soul. Physical characteristics of the body are determined by genetics and our bodies also may be predisposed to some mental and emotional states due to this, but it is the soul that puts the characteristics, the mental states, and mental acuity together to make you You.
In chapter 2, verse 17 of the Bhagvad Gita it is stated, ‘that which pervades the entire body is indestructible. No one can destroy the imperishable soul.” There is no living entity in this world that has not come into existence before. Every life form that exists now: plant, mammal, reptile has existed before in another form: bacteria, bird, or amphibian. Every creature goes through its respective cycle of rebirth trying to perfect their devotional service or bhakti that would liberate the soul from this cycle and return it to Godhead or Krishna to assume its natural and original nature: service to Krishna. Every soul has a purpose, every soul is some part of the divine haven and struggles every life and all life to attain that perfect state to return there.
Science tells us that humans are the most evolved of species the Bhagvad Gita further tells us that we are the most spiritually evolved species. Humans and animals engage in four common activities: eating, sleeping, mating and defending. What makes humans more highly evolved is consciousness: the ability to think, react, and rationalize. Although all living beings are souls part of the divine, the human state is the closest and necessary state to achieving liberation.
You have traveled across many births to have achieved this state of human hood. What will determine your progression or digression from or towards liberation is your bhakti and your karmic balance.