Solidarity is the firm and persevering determination to commit oneself to the common good — to the good of all and of each individual, because we are all truly responsible for all. Pope John Paul II developed this concept extensively in his 1987 encyclical Sollicitudo Rei Socialis, defining solidarity as 'not a feeling of vague compassion or shallow distress at the misfortunes of so many people' but rather 'a firm and persevering determination to commit oneself to the common good' (SRS 38). Solidarity recognizes that we are one human family, regardless of national, racial, ethnic, economic, or ideological differences. It calls us to see the 'other' — whether a neighbor or someone across the world — as our brother or sister. This principle demands concrete action: advocating for just economic structures, supporting fair trade, welcoming immigrants and refugees, and opposing systems that exploit the vulnerable. Solidarity is not optional charity but a moral obligation flowing from human dignity and our common humanity under God.