Is there any proof that humans reincarnate in animals? Or that ghost exist? I am very skeptical I’m sorry.
Don’t think of it as literal meanings. Remember, the Buddha taught using a lot of metaphors and parables, and using the way people understood rebirth back in the day (and according to the way Hindus believed), he used the six realms, but with much contemplation, you will realize the six realms are really six different states of mind.
So we have the 3 “fortunate” realms: human, asura (”demi-gods”) and deva (”heavenly beings”). Human is obvious, so I won’t go into that. Asuras are portrayed as beings addicted to passion, pride and envy. They are beings that have the means and the intentions of doing good, but end up committing bad actions and harming others. In a modern and real sense, Asuras are people who may be wealthy or in politics that do not contribute positively to the people and society. They are privileged people that go after power and control and do not stop for more.
Devas on the other hand, are pretty much the opposite of Asuras. They are generally happier, more peaceful, longer lived beings. In human form, they also could be wealthy or privileged people, but do not share the negative characteristics of Asuras. Instead, they are generous, compassionate and kind people who share their wealth or success with others.
Then we have the 3 “unfortunate” realms: hell, hungry ghosts and animals. Animals could be taken both literal and figuratively, because obviously there are real animals (some who act or think they’re human!) and then there are people who act like animals (with violence, anger, killing, etc.). To be reborn as an animal are people who were violent toward people and animals, harness anger and ignorance, and those with false views (i.e. believe that their truth is the absolute truth, even when it’s wrong).
Likewise, hell could also be taken literally or figuratively. At one point or another, we might all feel like we live in hell or in a hellish place. Our lives seem completely useless, everything is going wrong, nothing is in our favor, we constantly want more but can never have, etc. Hell is also a state of mind when we get so overwhelmed with anger and hate, that everything within and around us is chaotic. Beings reborn in hell are those who were greedy, selfish, ignorant, and those who feel that giving charity or donations is the hardest thing in the world.
When we see Buddhist art or images of hell beings or hungry ghosts, they are depicted as people with big bellies and a long, narrow neck and whenever they try to eat or drink something, it turns into flames and burns them. Because they were greedy and never wanted to give in their past lives, now they will not be able to get anything either.
Smile and be well!