Apologies for the terseness and grammatical liberties taken. These are even more like summary notes than usual.
“Be ye doers of the word” [from the Epistle, James 1:22-end]
St James' mirror analogy is a vivid portrayal of foolish forgetfulness. Seems absurd, but humans are capable of ignoring what they supposedly believe and what they have known from experience. Think of the goldfish swimming 'round its bowl, according to the urban myth about its 2 second memory: “Ooo, a rock! ... ... Ooo, a rock! ... ... Ooo, a rock! ... ...”
But it is only like forgetting, what the Epistle is addressing. The ideas are still in the mind, but are not in the heart and hands for the one whose “faith” is inactive.
Humans can also ignore and pretend not to know truths right in front of their faces, averting their gaze upon first sight. Or we can refuse to see them in the first place, closing our eyes. There are different species of spiritual blindness, but all cater to human failings, including pride, rebellion, hate, lust, greed, and laziness.
Imagine a man hearing and "appreciating" Golden Rule sermon, then driving home and abusing slow, elderly woman driver. “If you can’t drive, DON’T!” Unfortunately, this kind of thing happens. So, what should happen instead?
“Law of liberty” = free to do good. “Love and do what you please” says Augustine. This is sufficient if we allow the word of God to affect our minds AND WILLS. Don’t just agree to the truth, choose it. But we have been freed to be able to do this by faith in Christ. For the sincere disciple, there is freedom from guilt and fear, as sins are forgiven, and freedom from slavery to corruption, as God renews heart and mind. The law of liberty means good works are not a treadmill urging us on in futility, to earn God's favour, but an offering of hope from a position of favour/grace. True, penitent faith leads to action. It is itself obedience, according to St Paul, leading to more obedience: if it's real.
But what is “The real thing”? Self-control (speaking is the easiest action, but too easy, tongue-control = self-control in James 3); kindness, esp. to poor and needy; and not conforming to the ugliness and muck the world recommends. Speak less, help more, stay pure of heart. St James concentrates on our man-ward actions not because he disdains the necessity of worship, mystical experience of God or belief, for example [Cp. James 2:21 + 4:8a + 5:13], but because the latter are easier for us to perceive as deeper than they really are, as we attend to mere feelings and thoughts.
As we do the word, we allow God to change us more and more — and to make us happy (“blessed” in our “deed”).
And so we see that God corrects through his word common misconceptions (the default definitions of the World) of what being wise, free, religious and happy means. In opposition to the World, God says: Wisdom is worthless if not put into action. Freedom is empty if only an excuse for selfishness but full if it is freedom out of sin and in to the will of God. Religion is powerless and false if it is limited to elevated ideas and feelings but real and effective if it leads us to reflect God's love. Happiness that fulfills is derived partly from what we do for God and Mankind, and is not merely based on what happens to us or around us, that is, on pleasant circumstances.
So, as the Nike motto says, “Just do it”.