Assessing mental capacity is a serious exercise.

If you decide that a client lacks the capacity to do something you could deprive that client of a liberty.

If you decide that he or she has capacity you could normalize and empower that client.

But, by deciding that the client is capable, you might enable him or her to act, or carry on acting, in a manner which is prejudicial to their own best interests or those of others.

So how do you decide?Many of the answers can be found in Assessment of Mental Capacity: Guidanc e for Doctors and Lawyers which was published by The Law Society and the British Medical Association last week.One of the reasons why the two professional bodies joined forces to provide guidance for their members is illustrated by the following correspondence between a firm of solicitors and a GP.

'Dear [doctor], We act for [a client with learning difficulties] and would be grateful if you could let us know whether, in your opinion, she lacks capacity.

Yours faithfully, [solicitors].''Dear [solicitors], In my opinion [client] does not have capacity.

Yours faithfully, [doctor].'These letters are fictional but by no means atypical.

They contain two misconceptions.

First, they assume that anyone with any kind of mental disability is automatically incapable of making a decision for him or herself.

And, secondly, they regard incapacity as absolute.

No one, unless they are totally insensate, is completely incapacitated.

Over the years the law has laid down a number of different tests of capacity which reflect the enormity of the task to be performed or the decision to be made.

As Sir Owen Dixon, a former Chief Justice of Australia, put it: 'The mental capacity required by the law...is relative to the particular transaction which is being effected...and may be described as the capacity to understand the nature of that transaction when it is explained,' Gibbons v Wright [1954] 91 CLR 423, 438, applied in Re Beaney, deceased [1978] 2 All ER 595.The following solicitors' letter is slightly more sophisticated because it recognises that incapacity is specific rather than general.

'Dear [doctor], We act for [client] who, we understand, is suffering from the early stages of senile dementia.

She wishes to make a new will and we shall be grateful if you will let us have a report on whether or not she has testamentary capacity.

When submitting your report please would you let us have a note of your fee.

Yours faithfully, [solicitors].''Dear [solicitors], [client] has been suffering from dementia for the last six months and, in my opinion, does not have testamentary capacity.

I enclose a note of my fee.

Yours faithfully, [doctor].'Although the solicitors have requested an assessment of a specific type of capacity, they have not explained the legal requirements for assessing testamentary capacity -- the test in Banks v Goodfellow ([1870] LR5 QB 549) -- and they have neither told the doctor why the client wants to make a new will or supplied any information about the estate or family background.

The doctor in the above example does not seem to have examined the client specifically for the purpose of assessing testamentary capacity, has not explained why he or she believes that the client lacks capacity and has jumped to the conclusion that the diagnosis of a mental illness precludes the client from making a will a priori.In its report of March 1995 (Law Com No 231), which included a draft Mental Incapacity Bill, the Law Commission proposed that the secretary of state should prepare a code of practice 'for the guidance of persons assessing whether a person is or is not without capacity to make a decision or decisions on any matters' (cl 31).

Last week, the Lord Chancellor announced that he did not intend to legislate on the recommendations in their current form and said that there would be a further consultation (see [1995] Gazette, 17 January, 3).

If the government does not yet intend to legislate, the new guidance could be used as an exemplar of good practice for solicitors and doctors.WHERE A CLIENT MAY LACK CAPACITY:-- Act on the client's instructions or cease to act on the client's behalf.-- In borderline cases find out at the beginning whether the client has capacity or whether instructions must be given via a receiver, next friend or guardian ad litem.-- There is a legal presumption of capacity unless the contrary is shown.-- Different levels of capacity are required for different activities.-- The lawyer has a responsibility to explain to the doctor the relevant legal test of capability.-- Doctors and solicitors need to liaise most closely in marginal cases.

Fluctuating capacity poses problems for doctors.