Were the Pharisees purely "legalistic" thinkers? Or were they "discerning and critical thinkers"?
Jesus said that they had made God's word "invalid" because of their "tradition". That tradition involved interpretation of the law that went way above and beyond what God intended.
As an example, when Jesus and his disciples walked through a wheat field on the Sabbath and grabbed a few grains of wheat on their way through. (Luke 6:1-11) The Pharisees accused them of 'harvesting and threshing' on the Sabbath. And performing his healing miracles on the Sabbath was viewed with disdain as well.
The Sabbath was a day of rest from "work" and mundane chores. It was a day to devote to spiritual matters and reflection and meditation on spiritual things. If the son of God can grab a few grains of wheat without it being "work" or to heal the sick on their day of rest, then the Pharisees had nit-picked the law to death, legalistically speaking. Killing a flea on the Sabbath was considered "hunting". Seriously!
They were so concerned with the "letter of the Law" and their own rigid interpretation of it, that they lost sight of the spirit of it. They used the Talmud to go above the Law and accuse Jesus of Sabbath breaches and blasphemy.