It's impossible for someone to doubt their own sentience. The literal act of doubting is enough to dissipate all doubt. Solipsism is essentially the one certainty that every mind out there has.

Doubting the sentience of machines and even other humans is perfectly fine though. Only empathy allows people to make the leap and assume other humans have souls.

So you posit that humans are solipsistic by default, but some (most?) develop more and realize they’re not the only conscious being out there?