In the Catholic philosophical tradition, truth is the adequation of the mind with reality.
To know the truth is to say of what is that it is and of what is not that it is not.
The mind has the potential to be formed or in-formed by the form of things around us and by in-form-ation given to us by those whom we respect as authorities.
The form is what makes a thing to be what it is.
The mind grasps the form of things and abstracts the form from the matter that it forms.
What causes the thing to be what it is causes us to know what the thing is.
Catechism of the Catholic Church
I. LIVING IN THE TRUTH
2465 The Old Testament attests that God is the source of all truth. His Word is truth. His Law is truth. His "faithfulness endures to all generations."255 Since God is "true," the members of his people are called to live in the truth.256
2466 In Jesus Christ, the whole of God's truth has been made manifest. "Full of grace and truth," he came as the "light of the world," he is the Truth.257 "Whoever believes in me may not remain in darkness."258 The disciple of Jesus continues in his word so as to know "the truth [that] will make you free" and that sanctifies.259 To follow Jesus is to live in "the Spirit of truth," whom the Father sends in his name and who leads "into all the truth."260 To his disciples Jesus teaches the unconditional love of truth: "Let what you say be simply 'Yes or No.'"261
2467 Man tends by nature toward the truth. He is obliged to honor and bear witness to it: "It is in accordance with their dignity that all men, because they are persons . . . are both impelled by their nature and bound by a moral obligation to seek the truth, especially religious truth. They are also bound to adhere to the truth once they come to know it and direct their whole lives in accordance with the demands of truth."262
2468 Truth as uprightness in human action and speech is called truthfulness, sincerity, or candor. Truth or truthfulness is the virtue which consists in showing oneself true in deeds and truthful in words, and in guarding against duplicity, dissimulation, and hypocrisy.
2469 "Men could not live with one another if there were not mutual confidence that they were being truthful to one another."263 The virtue of truth gives another his just due. Truthfulness keeps to the just mean between what ought to be expressed and what ought to be kept secret: it entails honesty and discretion. In justice, "as a matter of honor, one man owes it to another to manifest the truth."264
2470 The disciple of Christ consents to "live in the truth," that is, in the simplicity of a life in conformity with the Lord's example, abiding in his truth. "If we say we have fellowship with him while we walk in darkness, we lie and do not live according to the truth."265