The law will never make men free; it is men who have got to make the law free. They are the lovers of law and order, who observe the law when the government breaks it.

Henry David Thoreau (1817-1862) American philosopher and writer

Speech (1854-07-04), “Slavery in Massachusetts,” Anti-Slavery Celebration, Framingham, Massachusetts
    (Source)

After the conviction in Boston of Anthony Burns, under the Fugitive Slave Act of 1850. This led to large protests and an abolitionist riot at the Boston Courthouse, requiring Federal troops and state militia to ensure Burns' transport to a ship sailing to Virginia.

In context, Thoreau is arguing the quality of a higher law, higher than the Fugitive Slave Law or Constitutional legalism from the courts -- the "law of humanity," which condemns the injustice of slavery.