The soul, practically speaking, is a hypothesis. It refers to 'that which animates a living creature.'
The spirit, practically speaking, is likewise a hypothesis. It refers to 'that which vanishes from a living creature once it ceases to live.' The remains--note the word--are what we can still see.
The two terms are often used as synonyms because after death the soul, minus its body, logically consists entirely of spirit. The two are essentially the same.
Early Christian writers, though, said the spirit would be united with another body after death. They were more or less obliged to take this position after making much of Christ's resurrection being a bodily one. Hence the fussiness many Christians still display over making a distinction between 'soul' and 'spirit'--even when they can't tell you what the distinction is.
Archer
All species are transitional.