One of the three requirements of a valid trust is the “certainty of the subject matter”,
There are two elements to certainty of subject matter:
- First, the property which is subject to the trust must be clear.
- Second, the nature of the interest due to each beneficiary must be clear.
To satisfy the first of the two elements, the subject matter must be described with “sufficient exactness to permit that such matter be ascertained at the time the trust was created”: Re Beardmore Trusts,  1 D.L.R. 41 at 46 (Ont. H.C.) [Beardmore].
Clearly, the subject matter must also be certain on future dates when the trustees are required to deal with the trust property. However, the trust property need not be fixed in quantity or nature; property can be added later, and the nature of existing property may be changed by the trustees exercising their power of investment. If the initial trust property is certain, and the property which may be added is certain, then the subject matter is certain because at any point the current trust property can be determined by tracing the original property to its current form: Waters at 155 -156.
The second element of certainty of subject matter requires that it be clear what beneficial share each beneficiary will receive in the trust property: Boyce v. Boyce (1949), 60 E.R. 959 (C.A.).