The field of activity is not determined by the nature of the planet influencing it.Nor is the nature of the planet determined by the field of activity. This is just like a person in a house. The person who happens to be in it does not determine the nature of the house, nor is the person determined by the nature of the house in which he resides. Both factors influence each other.
The person may alter the house and the house may limit the activity of the person. Hence, it is a question of compatibility or appropriateness of place and function.
The signs represent the field of activity. The planets represent the forces in operation within that field. It is important to remember the following rule:
A person with Taurus rising will have a Taurean field of activity or manifestation in the material world because Taurus is the sign of fixed earth. They will have a strong sense of matter, beauty and order. They will have a lot of work to do on a practical, material or physical level with the accumulation, development and refinement of form. Their lives will be oriented towards practical manifestation in the material world. Even when highly evolved spiritually, they will still have to manifest that spirituality through a mastery of the world of form. They will seek order, structure, beauty or love.
On the other hand, someone with a Libra Ascendant will be strongly oriented to the social world and toward the projection of ideas to change the world because Libra is cardinal air. They will be more socially, politically, group or friendship oriented and will like to be leaders in social or intellectual contexts. Their ideas and ideals will be more important to them than practical matters and they will be willing to sacrifice worldly goals and possessions for them.
The planets represent the kind of energy the individual will manifest within the field of activity represented by the sign. Taurus Ascendant strongly aspected by Saturn will give the individual concern with property, possessions, banking and a conservative trend of character or at least an extreme practicality in life and need for control and domination in the material world. Saturn's contracting nature will combine with the earthly qualities of Taurus.
Libra Ascendant strongly aspected by the Sun will make for a strong political, military, social or intellectual leader. The Libra field of activity will be dominated by the quality of the Sun.
Such are the discriminations that the astrologer must make in combining sign, house and planetary influences.
Generally, the ruler of the sign will have the strongest energy to influence it. If the ruler of a sign aspects its own sign or is strongly placed, the field and the force operating within the sign will be of the same character and strength. For example, if Aries is the Ascendant and Mars is in Leo, another fire sign, the properties of the Aries Ascendant will be increased. However, if Mars is in Virgo, a difficult sign for it, many of the properties of the Aries Ascendant may be weakened or inhibited. The individual may have ill health or difficulty in self-expression.
For example, Aries ascendant aspected by the Sun and Mars will be strong because both of these are fiery planets and friends. However, if it is aspected by natural and temporal malefics like Saturn and Mercury, which represent air and ether, it will be weakened.
Yet this general rule has exceptions. As the ruler of the sign is only one planet of nine, other planets commonly overcome its influence. For this reason, individuals may have a quality of energy or body type different than the predominant element of the Role.