One of the things everyone knows about the Catholic Church is that it teaches contraception is wrong.

The traditional teaching on contraception has complex sources, but is partly based on the story of Onan in the Old Testament, Genesis 38:7-10. The reasoning might be said to run as follows: Onan was condemned because he refused to be open to new life, also manifesting his lack of faith in God's providence. Therefore procreation was intrinsic to the meaning of the marital act; separating them changed its significance and offended God. Because of this it was unanimously held by Christians for centuries that it was sinful to dissociate the marital act from procreation, treating it as a means to pleasure alone.

This unanimity was first broken as late as 1930, by the Anglican Church, at the Lambeth Conference of that year. It was stated there that contraception was licit as long as it was done for moral reasons - thus implicitly assigning contraception itself no moral value, as other circumstances were held to be the deciding factor.

However, the Catholic Church has continued to teach that contraception is bad in itself, or "intrinsically disordered". The main source is Humanae Vitae, written by Pope Paul VI in 1968, which reiterated the age-old opposition to contraception. It states that there are two purposes of sexual intercourse: the "unitive" purpose and the "procreative" purpose. Contraception destroys the "procreative" purpose and is thus wrong.

The Catechism of the Catholic Church, written in 1994, also states this, and the Papal Encyclical Veritatis Splendor (1993) does so as well. According to this teaching, separation of the marital act from procreation is a sin, and thus contraception goes against human dignity. All these documents are authoritative. The last document mentioned used stronger language: "intrinsically evil".

All contraception is thus condemned, including of course the use of condoms, currently a topical subject. This is the background to statements such as "condoms change the beautiful act of love into a selfish search for pleasure, while rejecting responsibility" (Cardinal Trujillo, December 2003). It is true that if sexual relations and procreation are separated, certain types of behaviour are potentially capable of being acceptable that are not so if they are inseparable.

In recent years the Catholic Church has been criticised for this stance, and in particular it has been stated that because of this teaching hundreds (even thousands) of people are dying of Aids, usually, so it is said, in Africa. Presumably these people have scrupled to obey Catholic teaching in the relatively minor matter of contraception whilst at the same time completely ignoring it on the much more basic teaching re the absolute prohibition on sex outside marriage. It seems questionable.

The teaching on contraception has little to do with the Catholic Church's opposition to condoms in the fight against Aids, since there exists the principle of double effect: if the act is to save life rather than contraception, which is a side effect, it may be licit. Thus couples who are married may be able to protect themselves from HIV transmission in this way, if the marriage would otherwise be at risk. Other things being equal, abstinence may be the more sensible path as there is always a small risk of contracting a grave illness.

The difficulty has more to do with the use of the marital act outside marriage. As sexual relations outside marriage are (continuing the debate) outside the limits of what is moral, it makes less sense for moral norms to be set for them. In fact the Catholic Church has not made any authoritative pronouncement on them, although many articles have been published setting forth the general criteria. One of the more interesting pieces was written in the Catholic weekly magazine The Tablet. The author is a theologian who has worked for the Congregation for the Doctrine of the Faith.

As regards policies put into effect by government agencies and other organisations to combat the spread of HIV, Catholic leaders have been against (as they would see it) the bombarding of populations of young people with condoms, because it is thought they tend to give rise to increased promiscuity in the general population which at least partly compensates for the protection they grant. At least, that is the implication of surveys monitoring the spread of sexually transmitted diseases in areas where these policies are being operated, as reported in The Times ("Free pills and condoms boost promiscuity", 5 April 2004). This is sometimes put down to the need for more education, and isolated incidents might be ascribed to momentary stupidity; but sometimes it seems that the education being given is part of the problem. It seems to do little to encourage people to avoid becoming dependent on sex, unlike abstinence education.

This is the meaning of interventions such as that of Cardinal Trujillo, who has said that condoms do not guarantee protection against Aids. It has been suggested by some that with such phrases he and others are implying they are therefore no use at all, but as with other claims being made in this polemic, note should be taken of what was actually said. Likewise, few people would claim that abstinence education, proposed by Catholics and many others, is no use at all.