Any other vow, public or private, individual or collective, concerned with an action or with abstaining from an action, is a simple vow.
In canon law a vow is public (concerning the Church itself directly) only if a legitimate superior accepts it in the name of the Church; all other vows, no matter how much publicity is given to them, are classified as private vows (concerning directly only those who make them). The vow taken at profession as a member of any religious institute is a public vow, but in recent centuries can be either solemn or simple.
There is disagreement among theologians as to whether the distinction between solemn and simple vows derives simply from a decision of the Church to treat them differently or whether, in line with the opinion of Saint Thomas Aquinas, a solemn vow is, antecedently to any decision by the Church, a more strict, perfect and complete consecration to God.
Aquinas held that the only vows that could be considered solemn were those made by receiving holy orders or by the profession of the rule of a religious institute. As support for his view, he cited the fact that these two vows alone were considered to make the celebration of marriage invalid.
A man who promised, either to a human being or to God (thus making a vow), to marry a certain woman was bound by that promise or vow, but if he broke it and married a different woman, the marriage was nonetheless considered valid.
Similarly, if he made a vow to enter a particular religious institute or become a priest, but instead entered a different institute or decided to marry, the religious profession or the marriage, despite being a violation of his vow, was still considered valid. But once he had received holy orders or made religious profession, any marriage he contracted was considered null and void.
Solemn vows were originally considered indissoluble. Not even the Pope could dispense from them. If for a just cause a religious was expelled, the vow of chastity remained unchanged and so rendered invalid any attempt at marriage, the vow of obedience obliged in relation, generally, to the bishop rather than to the religious superior, and the vow of poverty was modified to meet the new situation but the expelled religious "could not, for example, will any goods to another; and goods which came to him reverted at his death to his institute or to the Holy See".