By 1844, the basic doctrinal structure of the Church was in place. Since that time, however, there have been official pronouncements clarifying doctrinal understanding or adapting doctrinal applications to particular circumstances. Some are now included in the Doctrine and Covenants; others are published as official messages of the First Presidency (cf. MFP). Over the years, various procedures and practices have received greater or lesser emphasis as changes have occurred in economic conditions (see Consecration; Tithing; United Order; Welfare), political circumstances (see Church and State; Politics; War and Peace), intellectual atmosphere (see Intellectual History), Church growth (see Organization), and many other areas. The essential doctrine of the Church, however, has remained constant amid such change. Certain Church leaders have written extensively of their understanding of the doctrines of the Church and, as a consequence, have had a significant influence on what many members believe (see treatises on doctrine below). These have included Parley P. Pratt, Orson Pratt, James E. Talmage, John A. Widtsoe, B. H. Roberts, Joseph Fielding Smith, and Bruce R. McConkie. Their writings evidence some differences of opinion on unsettled issues, just as different schools of thought exist among Church members in general on certain issues. Examples include efforts to reconcile current scientific teachings and revealed truths, to ponder the nature of uncreated intelligence, and to define eternal progression. Latter-day Saints have faith that answers will eventually be revealed, and are urged, in the meantime, to seek knowledge by all available means and to show tolerance toward those espousing differing opinions on such subjects.