Our brain activity is a matter of chemical transactions - not the sort above, bottles in the chemi lab at school, but tiny amounts, tiny quantitative changes. This is obvious, I guess - we all taker mood-altering chemicals: Pinot Noir, anti-depressants, best bitter, cannabis, etcetcetcetc. Our moods are a mix of reactions in the rest of our bodies, and reactions in our brains.
And our brains run on tiny electrical currents, connecting and disconnecting neurons all the time:
(I think I've got that about right so far, should have paid more attention at school. Mind you, that's so long ago that they taught us rather more about the stuff in the first picture than the second one.)
These facts have given rise to an unhelpful set of polarisations, which we might call romantic vs scientific. The first might be caricatured thus:
"It is reductive and mechanistic to say that we are only chemical reactions and electrical connections. Romeo says of Juliet
Oh, she doth teach the torches to burn bright!
It seems she hangs upon the cheek of night
Like a rich jewel in an Ethiope’s ear,
Beauty too rich for use, for earth too dear.
Note weasel-words "only" and "simply."